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Message-ID: <20211213105728.GA21139@linux.intel.com>
Date: Mon, 13 Dec 2021 18:57:29 +0800
From: Carel Si <beibei.si@...el.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: kernel test robot <oliver.sang@...el.com>,
Jann Horn <jannh@...gle.com>,
Miklos Szeredi <mszeredi@...hat.com>,
LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
kernel test robot <lkp@...el.com>, fengwei.yin@...el.com
Subject: Re: [LKP] Re: [fget] 054aa8d439: will-it-scale.per_thread_ops -5.7%
regression
Hi Linus,
On Fri, Dec 10, 2021 at 10:33:43AM -0800, Linus Torvalds wrote:
> On Thu, Dec 9, 2021 at 9:38 PM kernel test robot <oliver.sang@...el.com> wrote:
> >
> > FYI, we noticed a -5.7% regression of will-it-scale.per_thread_ops due to commit:
> > 054aa8d439b9 ("fget: check that the fd still exists after getting a ref to it")
>
> Well, some downside of the new checks was expected, that's just much
> more than I really like or would have thought.
>
> But it's exactly where you'd expect:
>
> > 27.16 ± 10% +4.3 31.51 ± 2% perf-profile.calltrace.cycles-pp.__fget_light.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
> > 22.91 ± 10% +4.4 27.34 ± 2% perf-profile.calltrace.cycles-pp.__fget_files.__fget_light.do_sys_poll.__x64_sys_poll.do_syscall_64
> > 26.33 ± 10% +4.4 30.70 ± 2% perf-profile.children.cycles-pp.__fget_light
> > 22.92 ± 10% +4.4 27.35 ± 2% perf-profile.children.cycles-pp.__fget_files
> > 22.70 ± 10% +4.4 27.11 ± 2% perf-profile.self.cycles-pp.__fget_files
>
> although there's odd spikes in dTLB-loads etc.
>
> I checked whether it's some unexpected code generation issue, but the
> new "re-check file table after refcount update" really looks very
> cheap when I look at what gcc generates, there's nothing really
> unexpected there.
>
> What did change was:
>
> (a) some branches go other ways, which might well affect branch
> prediction and just be unlucky. It might be that just marking the
> mismatch case "unlikely()" will help.
>
> (b) the obvious few new instructions (re-load and check file table
> pointer, re-load and check file pointer)
>
> (c) that __fget_files() function is now no longer a leaf function in
> a simple config case, since it calls "fput_many" in the error case.
>
> And that (c) is worth mentioning simply because it means that the
> function goes from not having any stack frame at all, to having to
> save/restore four registers. So now it has the usual push/pop
> sequences.
>
> It may also be that the test-case actually does a lot of threaded
> open/close/poll, and either actually triggers the re-lookup looping
> case (unlikely) or just sees a lot of cacheline bouncing that now got
> worse due to the re-check of the file pointer.
>
> So this regression looks real, and the issue seems to be that
> __fget_files() really is _that_ important for this do_sys_poll()
> benchmark, and even just the handful of extra instructions end up
> being meaningful.
>
> Oliver - I'm attaching the obvious "unlikely9)" oneliner in case it's
> just "gcc thought the retry loop was the common case" issue and bad
> branch prediction.
We tested your patch, it didn't work, still has -6.0% regression, thanks.
=========================================================================================
compiler/cpufreq_governor/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase/ucode:
gcc-9/performance/x86_64-rhel-8.3/thread/50%/debian-10.4-x86_64-20200603.cgz/lkp-ivb-2ep1/poll2/will-it-scale/0x42e
commit:
5f58da2bef ("Merge tag 'drm-fixes-2021-12-03-1' of git://anongit.freedesktop.org/drm/drm")
054aa8d439 ("fget: check that the fd still exists after getting a ref to it")
ef8c68873e ("fixup-for-054aa8d439")
5f58da2befa58edf 054aa8d439b9185d4f5eb9a9028 ef8c68873e75cf486bec22c3e8d
---------------- --------------------------- ---------------------------
%stddev %change %stddev %change %stddev
\ | \ | \
6666720 -5.7% 6288280 -6.0% 6268295 will-it-scale.24.threads
277779 -5.7% 262011 -6.0% 261178 will-it-scale.per_thread_ops
6666720 -5.7% 6288280 -6.0% 6268295 will-it-scale.workload
28.74 ± 23% -1.9 26.84 ± 6% +1.2 29.92 ± 25% perf-profile.calltrace.cycles-pp.intel_idle.cpuidle_enter_state.cpuidle_enter.do_idle.cpu_startup_entry
27.82 ± 23% -1.9 25.93 ± 5% +1.4 29.20 ± 26% perf-profile.calltrace.cycles-pp.cpuidle_enter.do_idle.cpu_startup_entry.start_secondary.secondary_startup_64_no_verify
27.82 ± 23% -1.9 25.93 ± 5% +1.4 29.20 ± 26% perf-profile.calltrace.cycles-pp.cpuidle_enter_state.cpuidle_enter.do_idle.cpu_startup_entry.start_secondary
27.83 ± 23% -1.9 25.94 ± 5% +1.4 29.20 ± 26% perf-profile.calltrace.cycles-pp.cpu_startup_entry.start_secondary.secondary_startup_64_no_verify
27.83 ± 23% -1.9 25.94 ± 5% +1.4 29.20 ± 26% perf-profile.calltrace.cycles-pp.do_idle.cpu_startup_entry.start_secondary.secondary_startup_64_no_verify
27.83 ± 23% -1.9 25.94 ± 5% +1.4 29.20 ± 26% perf-profile.calltrace.cycles-pp.start_secondary.secondary_startup_64_no_verify
28.81 ± 23% -1.9 26.93 ± 6% +1.2 29.98 ± 25% perf-profile.calltrace.cycles-pp.secondary_startup_64_no_verify
19.12 ± 9% -1.7 17.39 ± 2% -2.5 16.64 ± 11% perf-profile.calltrace.cycles-pp.fput_many.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
0.26 ±100% -0.2 0.10 ±223% -0.1 0.17 ±141% perf-profile.calltrace.cycles-pp.__kmalloc.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
2.88 ± 9% -0.1 2.76 ± 2% -0.2 2.64 ± 11% perf-profile.calltrace.cycles-pp.testcase
3.27 ± 9% -0.1 3.18 ± 2% -0.2 3.04 ± 11% perf-profile.calltrace.cycles-pp.syscall_exit_to_user_mode.do_syscall_64.entry_SYSCALL_64_after_hwframe.__poll
1.70 ± 9% -0.1 1.63 ± 2% -0.1 1.56 ± 11% perf-profile.calltrace.cycles-pp.__fdget.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
1.70 ± 10% -0.1 1.64 ± 2% -0.1 1.58 ± 10% perf-profile.calltrace.cycles-pp.fput.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
1.04 ± 8% -0.1 0.98 ± 4% -0.1 0.96 ± 11% perf-profile.calltrace.cycles-pp.__check_object_size.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
1.64 ± 9% -0.1 1.59 ± 3% -0.1 1.50 ± 10% perf-profile.calltrace.cycles-pp.__entry_text_start.__poll
1.69 ± 9% -0.0 1.65 ± 3% -0.1 1.57 ± 12% perf-profile.calltrace.cycles-pp.syscall_return_via_sysret.__poll
1.11 ± 9% -0.0 1.08 ± 4% -0.1 1.02 ± 9% perf-profile.calltrace.cycles-pp.copy_user_enhanced_fast_string._copy_from_user.do_sys_poll.__x64_sys_poll.do_syscall_64
1.48 ± 8% -0.0 1.45 ± 4% -0.1 1.38 ± 10% perf-profile.calltrace.cycles-pp._copy_from_user.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
0.94 ± 57% -0.0 0.92 ± 54% -0.2 0.74 ± 56% perf-profile.calltrace.cycles-pp.cpu_startup_entry.start_kernel.secondary_startup_64_no_verify
0.94 ± 57% -0.0 0.92 ± 54% -0.2 0.74 ± 56% perf-profile.calltrace.cycles-pp.do_idle.cpu_startup_entry.start_kernel.secondary_startup_64_no_verify
0.94 ± 57% -0.0 0.92 ± 54% -0.2 0.74 ± 56% perf-profile.calltrace.cycles-pp.cpuidle_enter.do_idle.cpu_startup_entry.start_kernel.secondary_startup_64_no_verify
0.94 ± 57% -0.0 0.92 ± 54% -0.2 0.74 ± 56% perf-profile.calltrace.cycles-pp.cpuidle_enter_state.cpuidle_enter.do_idle.cpu_startup_entry.start_kernel
0.94 ± 57% -0.0 0.92 ± 54% -0.2 0.74 ± 56% perf-profile.calltrace.cycles-pp.start_kernel.secondary_startup_64_no_verify
0.54 ± 45% +0.1 0.59 ± 9% +0.1 0.66 ± 6% perf-profile.calltrace.cycles-pp.kfree.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
67.66 ± 9% +2.0 69.69 ± 2% -0.9 66.78 ± 10% perf-profile.calltrace.cycles-pp.__poll
63.66 ± 9% +2.2 65.82 ± 2% -0.6 63.10 ± 10% perf-profile.calltrace.cycles-pp.entry_SYSCALL_64_after_hwframe.__poll
63.48 ± 9% +2.2 65.64 ± 2% -0.6 62.93 ± 10% perf-profile.calltrace.cycles-pp.do_syscall_64.entry_SYSCALL_64_after_hwframe.__poll
59.85 ± 9% +2.3 62.12 ± 2% -0.3 59.56 ± 10% perf-profile.calltrace.cycles-pp.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe.__poll
57.74 ± 9% +2.4 60.09 ± 2% -0.1 57.62 ± 10% perf-profile.calltrace.cycles-pp.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe.__poll
27.16 ± 10% +4.3 31.51 ± 2% +3.0 30.21 ± 10% perf-profile.calltrace.cycles-pp.__fget_light.do_sys_poll.__x64_sys_poll.do_syscall_64.entry_SYSCALL_64_after_hwframe
22.91 ± 10% +4.4 27.34 ± 2% +3.3 26.25 ± 10% perf-profile.calltrace.cycles-pp.__fget_files.__fget_light.do_sys_poll.__x64_sys_poll.do_syscall_64
27.83 ± 23% -1.9 25.94 ± 5% +1.4 29.20 ± 26% perf-profile.children.cycles-pp.start_secondary
28.80 ± 23% -1.9 26.92 ± 6% +1.2 29.98 ± 25% perf-profile.children.cycles-pp.cpuidle_enter
28.80 ± 23% -1.9 26.92 ± 6% +1.2 29.98 ± 25% perf-profile.children.cycles-pp.cpuidle_enter_state
28.81 ± 23% -1.9 26.93 ± 6% +1.2 29.98 ± 25% perf-profile.children.cycles-pp.secondary_startup_64_no_verify
28.81 ± 23% -1.9 26.93 ± 6% +1.2 29.98 ± 25% perf-profile.children.cycles-pp.cpu_startup_entry
28.81 ± 23% -1.9 26.93 ± 6% +1.2 29.98 ± 25% perf-profile.children.cycles-pp.do_idle
28.78 ± 23% -1.9 26.91 ± 6% +1.2 29.96 ± 25% perf-profile.children.cycles-pp.intel_idle
18.28 ± 9% -1.7 16.59 ± 2% -2.4 15.87 ± 11% perf-profile.children.cycles-pp.fput_many
2.90 ± 9% -0.1 2.76 ± 2% -0.3 2.64 ± 11% perf-profile.children.cycles-pp.testcase
3.29 ± 9% -0.1 3.20 ± 2% -0.2 3.07 ± 11% perf-profile.children.cycles-pp.syscall_exit_to_user_mode
1.69 ± 9% -0.1 1.63 ± 2% -0.1 1.57 ± 10% perf-profile.children.cycles-pp.__fdget
1.70 ± 9% -0.1 1.64 ± 2% -0.1 1.57 ± 10% perf-profile.children.cycles-pp.fput
1.82 ± 9% -0.1 1.76 ± 3% -0.2 1.67 ± 11% perf-profile.children.cycles-pp.__entry_text_start
1.07 ± 9% -0.1 1.02 ± 4% -0.1 0.99 ± 10% perf-profile.children.cycles-pp.__check_object_size
1.90 ± 9% -0.0 1.86 ± 2% -0.1 1.77 ± 12% perf-profile.children.cycles-pp.syscall_return_via_sysret
1.50 ± 8% -0.0 1.47 ± 4% -0.1 1.39 ± 10% perf-profile.children.cycles-pp._copy_from_user
1.12 ± 9% -0.0 1.09 ± 4% -0.1 1.03 ± 10% perf-profile.children.cycles-pp.copy_user_enhanced_fast_string
0.30 ± 8% -0.0 0.26 ± 11% -0.0 0.29 ± 20% perf-profile.children.cycles-pp.__virt_addr_valid
0.12 ± 8% -0.0 0.09 ± 11% -0.0 0.09 ± 12% perf-profile.children.cycles-pp.poll@plt
0.04 ± 71% -0.0 0.01 ±223% -0.0 0.03 ±100% perf-profile.children.cycles-pp.seq_read_iter
0.17 ± 26% -0.0 0.14 ± 32% -0.0 0.14 ± 23% perf-profile.children.cycles-pp.cmd_sched
0.17 ± 26% -0.0 0.14 ± 32% -0.0 0.14 ± 23% perf-profile.children.cycles-pp.cmd_record
0.04 ± 71% -0.0 0.02 ±144% -0.0 0.03 ±100% perf-profile.children.cycles-pp.ksys_read
0.04 ± 71% -0.0 0.02 ±144% -0.0 0.03 ±100% perf-profile.children.cycles-pp.vfs_read
0.61 ± 13% -0.0 0.59 ± 9% +0.1 0.66 ± 5% perf-profile.children.cycles-pp.kfree
0.17 ± 26% -0.0 0.15 ± 30% -0.0 0.15 ± 21% perf-profile.children.cycles-pp.__libc_start_main
0.17 ± 26% -0.0 0.15 ± 30% -0.0 0.15 ± 21% perf-profile.children.cycles-pp.main
0.17 ± 26% -0.0 0.15 ± 30% -0.0 0.15 ± 21% perf-profile.children.cycles-pp.run_builtin
0.02 ±142% -0.0 0.00 -0.0 0.00 perf-profile.children.cycles-pp.new_sync_write
0.02 ±142% -0.0 0.00 -0.0 0.01 ±223% perf-profile.children.cycles-pp.ksys_write
0.02 ±142% -0.0 0.00 -0.0 0.01 ±223% perf-profile.children.cycles-pp.vfs_write
0.04 ± 45% -0.0 0.02 ± 99% -0.0 0.02 ± 99% perf-profile.children.cycles-pp.__x86_indirect_thunk_rax
0.13 ± 10% -0.0 0.12 ± 6% -0.0 0.12 ± 15% perf-profile.children.cycles-pp.kmalloc_slab
0.02 ± 99% -0.0 0.01 ±223% +0.0 0.02 ± 99% perf-profile.children.cycles-pp.seq_read
0.02 ± 99% -0.0 0.01 ±223% +0.0 0.02 ± 99% perf-profile.children.cycles-pp.__libc_read
0.15 ± 26% -0.0 0.14 ± 31% -0.0 0.13 ± 23% perf-profile.children.cycles-pp.record__finish_output
0.15 ± 26% -0.0 0.14 ± 31% -0.0 0.13 ± 23% perf-profile.children.cycles-pp.perf_session__process_events
0.06 ± 46% -0.0 0.04 ± 73% -0.0 0.04 ± 73% perf-profile.children.cycles-pp.machines__deliver_event
0.14 ± 27% -0.0 0.12 ± 32% -0.0 0.12 ± 19% perf-profile.children.cycles-pp.process_simple
0.09 ± 30% -0.0 0.08 ± 34% -0.0 0.08 ± 17% perf-profile.children.cycles-pp.perf_session__process_user_event
0.05 ± 47% -0.0 0.04 ± 71% -0.0 0.02 ±142% perf-profile.children.cycles-pp.exc_page_fault
0.10 ± 24% -0.0 0.08 ± 30% -0.0 0.08 ± 17% perf-profile.children.cycles-pp.perf_session__deliver_event
0.02 ±142% -0.0 0.01 ±223% -0.0 0.01 ±223% perf-profile.children.cycles-pp.poll_select_set_timeout
0.09 ± 24% -0.0 0.08 ± 28% -0.0 0.08 ± 19% perf-profile.children.cycles-pp.__ordered_events__flush
0.02 ±141% -0.0 0.01 ±223% -0.0 0.00 perf-profile.children.cycles-pp.proc_reg_read
0.02 ±141% -0.0 0.01 ±223% -0.0 0.01 ±223% perf-profile.children.cycles-pp.perf_output_sample
0.01 ±223% -0.0 0.00 +0.0 0.01 ±223% perf-profile.children.cycles-pp.do_user_addr_fault
0.04 ± 45% -0.0 0.04 ± 71% -0.0 0.00 perf-profile.children.cycles-pp.perf_callchain_user
0.29 ± 9% -0.0 0.28 ± 4% -0.0 0.28 ± 10% perf-profile.children.cycles-pp.entry_SYSCALL_64_safe_stack
0.11 ± 12% -0.0 0.10 ± 8% -0.0 0.10 ± 10% perf-profile.children.cycles-pp.__might_sleep
0.05 ± 49% -0.0 0.05 ± 52% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.check_stack_object
0.16 ± 20% -0.0 0.16 ± 9% -0.0 0.12 ± 11% perf-profile.children.cycles-pp.perf_callchain_kernel
0.12 ± 20% -0.0 0.11 ± 8% -0.0 0.10 ± 11% perf-profile.children.cycles-pp.unwind_next_frame
0.29 ± 11% -0.0 0.28 ± 5% -0.0 0.26 ± 9% perf-profile.children.cycles-pp.__check_heap_object
0.43 ± 19% +0.0 0.43 ± 10% -0.1 0.37 ± 9% perf-profile.children.cycles-pp.update_process_times
0.23 ± 20% +0.0 0.23 ± 11% -0.0 0.18 ± 9% perf-profile.children.cycles-pp.perf_prepare_sample
0.21 ± 20% +0.0 0.21 ± 10% -0.0 0.17 ± 10% perf-profile.children.cycles-pp.perf_callchain
0.02 ±142% +0.0 0.02 ±142% +0.0 0.02 ±142% perf-profile.children.cycles-pp.io_serial_in
0.02 ±141% +0.0 0.02 ±141% +0.0 0.02 ±141% perf-profile.children.cycles-pp.ordered_events__queue
0.07 ± 12% +0.0 0.07 ± 6% -0.0 0.07 ± 10% perf-profile.children.cycles-pp.uart_console_write
0.07 ± 12% +0.0 0.08 ± 6% -0.0 0.07 ± 10% perf-profile.children.cycles-pp.serial8250_console_write
0.21 ± 21% +0.0 0.21 ± 10% -0.0 0.17 ± 10% perf-profile.children.cycles-pp.get_perf_callchain
0.07 ± 12% +0.0 0.07 ± 6% -0.0 0.07 ± 10% perf-profile.children.cycles-pp.wait_for_xmitr
0.08 ± 12% +0.0 0.08 ± 6% -0.0 0.07 ± 12% perf-profile.children.cycles-pp.vprintk_emit
0.08 ± 12% +0.0 0.08 ± 6% -0.0 0.07 ± 12% perf-profile.children.cycles-pp.console_unlock
0.48 ± 17% +0.0 0.48 ± 9% -0.1 0.42 ± 10% perf-profile.children.cycles-pp.__hrtimer_run_queues
0.32 ± 21% +0.0 0.32 ± 11% -0.1 0.26 ± 11% perf-profile.children.cycles-pp.update_curr
0.61 ± 16% +0.0 0.61 ± 7% -0.1 0.55 ± 10% perf-profile.children.cycles-pp.asm_sysvec_apic_timer_interrupt
0.30 ± 21% +0.0 0.30 ± 10% -0.1 0.24 ± 10% perf-profile.children.cycles-pp.perf_trace_sched_stat_runtime
0.28 ± 20% +0.0 0.29 ± 10% -0.1 0.23 ± 11% perf-profile.children.cycles-pp.__perf_event_overflow
0.28 ± 19% +0.0 0.28 ± 11% -0.1 0.23 ± 10% perf-profile.children.cycles-pp.perf_event_output_forward
0.29 ± 10% +0.0 0.29 ± 9% -0.0 0.28 ± 10% perf-profile.children.cycles-pp.__might_fault
0.08 ± 12% +0.0 0.08 ± 22% -0.0 0.07 ± 14% perf-profile.children.cycles-pp.syscall_enter_from_user_mode
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.worker_thread
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.process_one_work
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.drm_fb_helper_damage_work
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.drm_atomic_helper_dirtyfb
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.drm_atomic_helper_commit
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.commit_tail
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.drm_atomic_helper_commit_tail
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.drm_atomic_helper_commit_planes
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.mgag200_simple_display_pipe_update
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.drm_fb_memcpy_dstclip
0.05 ± 8% +0.0 0.06 ± 13% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.memcpy_toio
0.07 ± 8% +0.0 0.07 ± 6% -0.0 0.07 ± 10% perf-profile.children.cycles-pp.serial8250_console_putchar
0.43 ± 19% +0.0 0.44 ± 9% -0.1 0.38 ± 9% perf-profile.children.cycles-pp.tick_sched_handle
0.57 ± 16% +0.0 0.58 ± 7% -0.1 0.52 ± 9% perf-profile.children.cycles-pp.sysvec_apic_timer_interrupt
0.05 ± 47% +0.0 0.06 ± 8% -0.0 0.04 ± 71% perf-profile.children.cycles-pp.__unwind_start
0.28 ± 20% +0.0 0.29 ± 10% -0.1 0.23 ± 11% perf-profile.children.cycles-pp.perf_swevent_overflow
0.07 ± 9% +0.0 0.08 ± 6% -0.0 0.07 ± 11% perf-profile.children.cycles-pp.asm_sysvec_irq_work
0.07 ± 9% +0.0 0.08 ± 6% -0.0 0.07 ± 11% perf-profile.children.cycles-pp.sysvec_irq_work
0.07 ± 9% +0.0 0.08 ± 6% -0.0 0.07 ± 11% perf-profile.children.cycles-pp.__sysvec_irq_work
0.07 ± 9% +0.0 0.08 ± 6% -0.0 0.07 ± 11% perf-profile.children.cycles-pp.irq_work_run
0.07 ± 9% +0.0 0.08 ± 6% -0.0 0.07 ± 11% perf-profile.children.cycles-pp.irq_work_single
0.07 ± 9% +0.0 0.08 ± 6% -0.0 0.07 ± 11% perf-profile.children.cycles-pp._printk
0.44 ± 19% +0.0 0.45 ± 9% -0.1 0.38 ± 9% perf-profile.children.cycles-pp.tick_sched_timer
0.54 ± 17% +0.0 0.55 ± 8% -0.1 0.49 ± 10% perf-profile.children.cycles-pp.__sysvec_apic_timer_interrupt
0.54 ± 17% +0.0 0.54 ± 8% -0.1 0.48 ± 10% perf-profile.children.cycles-pp.hrtimer_interrupt
0.37 ± 19% +0.0 0.37 ± 10% -0.1 0.31 ± 9% perf-profile.children.cycles-pp.task_tick_fair
0.06 ± 47% +0.0 0.07 ± 18% -0.0 0.06 ± 21% perf-profile.children.cycles-pp.asm_exc_page_fault
0.07 ± 9% +0.0 0.08 ± 8% -0.0 0.07 ± 11% perf-profile.children.cycles-pp.irq_work_run_list
0.98 ± 47% +0.0 0.99 ± 38% -0.2 0.78 ± 45% perf-profile.children.cycles-pp.start_kernel
0.29 ± 21% +0.0 0.30 ± 10% -0.1 0.24 ± 10% perf-profile.children.cycles-pp.perf_tp_event
0.01 ±223% +0.0 0.02 ±141% +0.0 0.01 ±223% perf-profile.children.cycles-pp.queue_event
0.00 +0.0 0.01 ±223% +0.0 0.01 ±223% perf-profile.children.cycles-pp.build_id__mark_dso_hit
0.01 ±223% +0.0 0.02 ±141% +0.0 0.02 ± 99% perf-profile.children.cycles-pp.__cond_resched
0.00 +0.0 0.01 ±223% +0.0 0.02 ± 99% perf-profile.children.cycles-pp.poll_freewait
0.39 ± 19% +0.0 0.40 ± 9% -0.1 0.34 ± 9% perf-profile.children.cycles-pp.scheduler_tick
0.04 ± 71% +0.0 0.04 ± 45% -0.0 0.02 ±141% perf-profile.children.cycles-pp.exit_to_user_mode_prepare
0.06 ± 8% +0.0 0.07 ± 11% +0.0 0.06 ± 13% perf-profile.children.cycles-pp.ret_from_fork
0.06 ± 8% +0.0 0.07 ± 11% +0.0 0.06 ± 13% perf-profile.children.cycles-pp.kthread
0.19 ± 10% +0.0 0.20 ± 9% +0.0 0.19 ± 10% perf-profile.children.cycles-pp.__might_resched
0.02 ±141% +0.0 0.03 ± 70% -0.0 0.00 perf-profile.children.cycles-pp.syscall_exit_to_user_mode_prepare
0.01 ±223% +0.0 0.03 ± 70% -0.0 0.00 perf-profile.children.cycles-pp.__get_user_nocheck_8
0.50 ± 12% +0.0 0.53 ± 9% -0.0 0.48 ± 11% perf-profile.children.cycles-pp.__kmalloc
67.89 ± 9% +2.0 69.92 ± 2% -0.9 67.00 ± 10% perf-profile.children.cycles-pp.__poll
63.84 ± 9% +2.1 65.97 ± 2% -0.6 63.26 ± 10% perf-profile.children.cycles-pp.entry_SYSCALL_64_after_hwframe
63.66 ± 9% +2.1 65.80 ± 2% -0.6 63.09 ± 10% perf-profile.children.cycles-pp.do_syscall_64
59.86 ± 9% +2.3 62.12 ± 2% -0.3 59.57 ± 10% perf-profile.children.cycles-pp.__x64_sys_poll
59.47 ± 9% +2.3 61.75 ± 2% -0.2 59.22 ± 10% perf-profile.children.cycles-pp.do_sys_poll
26.33 ± 10% +4.4 30.70 ± 2% +3.1 29.43 ± 10% perf-profile.children.cycles-pp.__fget_light
22.92 ± 10% +4.4 27.35 ± 2% +3.3 26.26 ± 10% perf-profile.children.cycles-pp.__fget_files
28.78 ± 23% -1.9 26.91 ± 6% +1.2 29.96 ± 25% perf-profile.self.cycles-pp.intel_idle
17.29 ± 9% -1.6 15.64 ± 2% -2.3 14.96 ± 11% perf-profile.self.cycles-pp.fput_many
11.06 ± 9% -0.3 10.74 ± 2% -0.8 10.29 ± 10% perf-profile.self.cycles-pp.do_sys_poll
2.87 ± 9% -0.1 2.74 ± 2% -0.2 2.62 ± 11% perf-profile.self.cycles-pp.testcase
3.21 ± 9% -0.1 3.12 ± 2% -0.2 3.00 ± 11% perf-profile.self.cycles-pp.syscall_exit_to_user_mode
1.60 ± 9% -0.1 1.54 ± 4% -0.1 1.47 ± 10% perf-profile.self.cycles-pp.__entry_text_start
1.90 ± 8% -0.0 1.86 ± 3% -0.1 1.76 ± 12% perf-profile.self.cycles-pp.syscall_return_via_sysret
2.55 ± 9% -0.0 2.51 ± 4% -0.2 2.36 ± 12% perf-profile.self.cycles-pp.__fget_light
0.84 ± 8% -0.0 0.81 ± 2% -0.1 0.78 ± 10% perf-profile.self.cycles-pp.fput
0.29 ± 8% -0.0 0.26 ± 11% -0.0 0.28 ± 20% perf-profile.self.cycles-pp.__virt_addr_valid
1.10 ± 9% -0.0 1.07 ± 4% -0.1 1.01 ± 10% perf-profile.self.cycles-pp.copy_user_enhanced_fast_string
0.28 ± 8% -0.0 0.26 ± 5% -0.0 0.26 ± 11% perf-profile.self.cycles-pp.do_syscall_64
0.84 ± 9% -0.0 0.82 ± 3% -0.0 0.79 ± 10% perf-profile.self.cycles-pp.__fdget
0.10 ± 6% -0.0 0.08 ± 10% -0.0 0.08 ± 14% perf-profile.self.cycles-pp.poll@plt
0.60 ± 12% -0.0 0.59 ± 9% +0.1 0.66 ± 6% perf-profile.self.cycles-pp.kfree
0.11 ± 12% -0.0 0.09 ± 7% -0.0 0.10 ± 13% perf-profile.self.cycles-pp._copy_from_user
0.44 ± 9% -0.0 0.43 ± 3% -0.0 0.40 ± 15% perf-profile.self.cycles-pp.__check_object_size
0.31 ± 10% -0.0 0.30 ± 3% -0.0 0.28 ± 15% perf-profile.self.cycles-pp.__x64_sys_poll
0.18 ± 8% -0.0 0.17 ± 5% -0.0 0.16 ± 11% perf-profile.self.cycles-pp.entry_SYSCALL_64_after_hwframe
0.02 ±142% -0.0 0.01 ±223% -0.0 0.01 ±223% perf-profile.self.cycles-pp.poll_select_set_timeout
0.04 ± 71% -0.0 0.02 ± 99% -0.0 0.02 ± 99% perf-profile.self.cycles-pp.__x86_indirect_thunk_rax
0.12 ± 10% -0.0 0.12 ± 6% -0.0 0.12 ± 17% perf-profile.self.cycles-pp.kmalloc_slab
0.01 ±223% -0.0 0.00 -0.0 0.00 perf-profile.self.cycles-pp.syscall_exit_to_user_mode_prepare
0.06 ± 11% -0.0 0.05 ± 8% -0.0 0.04 ± 71% perf-profile.self.cycles-pp.syscall_enter_from_user_mode
0.03 ±100% -0.0 0.02 ±142% -0.0 0.02 ±141% perf-profile.self.cycles-pp.check_stack_object
0.28 ± 11% -0.0 0.27 ± 5% -0.0 0.25 ± 11% perf-profile.self.cycles-pp.__check_heap_object
0.09 ± 13% -0.0 0.09 ± 7% -0.0 0.09 ± 10% perf-profile.self.cycles-pp.__might_sleep
0.08 ± 17% -0.0 0.08 ± 25% -0.0 0.08 ± 11% perf-profile.self.cycles-pp.__might_fault
0.03 ±100% -0.0 0.02 ± 99% -0.0 0.02 ±141% perf-profile.self.cycles-pp.unwind_next_frame
0.01 ±223% +0.0 0.01 ±223% -0.0 0.00 perf-profile.self.cycles-pp.exit_to_user_mode_prepare
0.02 ±142% +0.0 0.02 ±142% +0.0 0.02 ±142% perf-profile.self.cycles-pp.io_serial_in
0.11 ± 9% +0.0 0.12 ± 8% -0.0 0.11 ± 11% perf-profile.self.cycles-pp.entry_SYSCALL_64_safe_stack
0.44 ± 8% +0.0 0.45 ± 4% -0.0 0.42 ± 12% perf-profile.self.cycles-pp.__poll
0.01 ±223% +0.0 0.02 ±141% +0.0 0.01 ±223% perf-profile.self.cycles-pp.queue_event
0.18 ± 11% +0.0 0.19 ± 8% -0.0 0.18 ± 10% perf-profile.self.cycles-pp.__might_resched
0.26 ± 13% +0.0 0.30 ± 17% -0.0 0.25 ± 13% perf-profile.self.cycles-pp.__kmalloc
22.70 ± 10% +4.4 27.11 ± 2% +3.3 26.04 ± 10% perf-profile.self.cycles-pp.__fget_files
>
> And it would perhaps be interesting to get an actual instruction-level
> profile of that __fget_files() thing for that benchmark, if that
> pinpoints exactly what is going on and in case that would be easy to
> get on that machine.
>
> Because it might just be truly horrendously bad luck, with the 32-byte
> stack frame meaning that the kernel stack goes one more page down
> (just jhandwaving from the dTLB number spike), and this all being just
> random bad luck on that particular benchmark.
>
> Of course, the thing about poll() is that for that case, we *don't*
> really need the "re-check the file descriptor" code at all, since the
> resulting fd isn't going to be installed as a new fd, and it doesn't
> matter for the socket garbage collector logic.
>
> So maybe it was a mistake to put that re-check in the generic fdget()
> code - yes, it should be cheap, but it's also some of the most hot
> code in the kernel on some loads.
>
> But if we move it elsewhere, we'd need to come up with some list of
> "these cases need it". Some are obvious: dup, dup2, unix domain file
> passing. It's the non-obvious ones I'd worry about.
>
> Anybody?
>
> Linus
> fs/file.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/file.c b/fs/file.c
> index ad4a8bf3cf10..f802360e240d 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -858,7 +858,7 @@ static struct file *__fget_files(struct files_struct *files, unsigned int fd,
> file = NULL;
> else if (!get_file_rcu_many(file, refs))
> goto loop;
> - else if (files_lookup_fd_raw(files, fd) != file) {
> + else if (unlikely(files_lookup_fd_raw(files, fd) != file)) {
> fput_many(file, refs);
> goto loop;
> }
> _______________________________________________
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