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Message-ID: <CABPqkBSuQoedYqe7EZ567vD4eTfxbzsLJhGJoGUh0d0GPJ=O0A@mail.gmail.com> Date: Sun, 23 Feb 2014 20:47:55 +0100 From: Stephane Eranian <eranian@...gle.com> To: "Yan, Zheng" <zheng.z.yan@...el.com> Cc: LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Ingo Molnar <mingo@...nel.org>, Arnaldo Carvalho de Melo <acme@...radead.org>, Andi Kleen <andi@...stfloor.org> Subject: Re: [PATCH v3 00/14] perf, x86: Haswell LBR call stack support Could you add the Reviewed-by: on the patches I already reviewed? So I focus on the changes you made and continue testing on my HSW system. Thanks. On Tue, Feb 18, 2014 at 7:07 AM, Yan, Zheng <zheng.z.yan@...el.com> wrote: > For many profiling tasks we need the callgraph. For example we often > need to see the caller of a lock or the caller of a memcpy or other > library function to actually tune the program. Frame pointer unwinding > is efficient and works well. But frame pointers are off by default on > 64bit code (and on modern 32bit gccs), so there are many binaries around > that do not use frame pointers. Profiling unchanged production code is > very useful in practice. On some CPUs frame pointer also has a high > cost. Dwarf2 unwinding also does not always work and is extremely slow > (upto 20% overhead). > > Haswell has a new feature that utilizes the existing Last Branch Record > facility to record call chains. When the feature is enabled, function > call will be collected as normal, but as return instructions are > executed the last captured branch record is popped from the on-chip LBR > registers. The LBR call stack facility provides an alternative to get > callgraph. It has some limitations too, but should work in most cases > and is significantly faster than dwarf. Frame pointer unwinding is still > the best default, but LBR call stack is a good alternative when nothing > else works. > > This patch series adds LBR call stack support. User can enabled/disable > this through an sysfs attribute file in the CPU PMU directory: > echo 1 > /sys/bus/event_source/devices/cpu/lbr_callstack > > When profiling bc(1) on Fedora 19: > echo 'scale=2000; 4*a(1)' > cmd; perf record -g fp bc -l < cmd > > If this feature is enabled, perf report output looks like: > 50.36% bc bc [.] bc_divide > | > --- bc_divide > execute > run_code > yyparse > main > __libc_start_main > _start > > 33.66% bc bc [.] _one_mult > | > --- _one_mult > bc_divide > execute > run_code > yyparse > main > __libc_start_main > _start > > 7.62% bc bc [.] _bc_do_add > | > --- _bc_do_add > | > |--99.89%-- 0x2000186a8 > --0.11%-- [...] > > 6.83% bc bc [.] _bc_do_sub > | > --- _bc_do_sub > | > |--99.94%-- bc_add > | execute > | run_code > | yyparse > | main > | __libc_start_main > | _start > --0.06%-- [...] > > 0.46% bc libc-2.17.so [.] __memset_sse2 > | > --- __memset_sse2 > | > |--54.13%-- bc_new_num > | | > | |--51.00%-- bc_divide > | | execute > | | run_code > | | yyparse > | | main > | | __libc_start_main > | | _start > | | > | |--30.46%-- _bc_do_sub > | | bc_add > | | execute > | | run_code > | | yyparse > | | main > | | __libc_start_main > | | _start > | | > | --18.55%-- _bc_do_add > | bc_add > | execute > | run_code > | yyparse > | main > | __libc_start_main > | _start > | > --45.87%-- bc_divide > execute > run_code > yyparse > main > __libc_start_main > _start > > If this feature is disabled, perf report output looks like: > 50.49% bc bc [.] bc_divide > | > --- bc_divide > > 33.57% bc bc [.] _one_mult > | > --- _one_mult > > 7.61% bc bc [.] _bc_do_add > | > --- _bc_do_add > 0x2000186a8 > > 6.88% bc bc [.] _bc_do_sub > | > --- _bc_do_sub > > 0.42% bc libc-2.17.so [.] __memcpy_ssse3_back > | > --- __memcpy_ssse3_back > > The LBR call stack has following known limitations > - Zero length calls are not filtered out by hardware > - Exception handing such as setjmp/longjmp will have calls/returns not > match > - Pushing different return address onto the stack will have calls/returns > not match > - If callstack is deeper than the LBR, only the last entries are captured > > Changes since v1 > - split change into more patches > - introduce context switch callback and use it to flush LBR > - use the context switch callback to save/restore LBR > - dynamic allocate memory area for storing LBR stack, always switch the > memory area during context switch > - disable this feature by default > - more description in change logs > > Changes since v2 > - don't use xchg to switch PMU specific data > - remove nr_branch_stack from struct perf_event_context > - simplify the save/restore LBR stack logical > - remove unnecessary 'has_branch_stack -> needs_branch_stack' > conversion > - more description in change logs > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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