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Message-ID: <20131025163303.GD108330@redhat.com>
Date: Fri, 25 Oct 2013 12:33:03 -0400
From: Don Zickus <dzickus@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Andi Kleen <ak@...ux.intel.com>, dave.hansen@...ux.intel.com,
Stephane Eranian <eranian@...gle.com>, jmario@...hat.com,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Arnaldo Carvalho de Melo <acme@...radead.org>,
Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()
On Thu, Oct 24, 2013 at 12:52:06PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 23, 2013 at 10:48:38PM +0200, Peter Zijlstra wrote:
> > I'll also make sure to test we actually hit the fault path
> > by concurrently running something like:
> >
> > while :; echo 1 > /proc/sys/vm/drop_caches ; done
> >
> > while doing perf top or so..
>
> So the below appears to work; I've ran:
>
> while :; do echo 1 > /proc/sys/vm/drop_caches; sleep 1; done &
> while :; do make O=defconfig-build/ clean; perf record -a -g fp -e cycles:pp make O=defconfig-build/ -s -j64; done
>
> And verified that the if (in_nmi()) trace_printk() was visible in the
> trace output verifying we indeed took the fault from the NMI code.
>
> I've had this running for ~ 30 minutes or so and the machine is still
> healthy.
>
> Don, can you give this stuff a spin on your system?
Hi Peter,
I finally had a chance to run this on my machine. From my testing, it
looks good. Better performance numbers. I think my longest latency went
from 300K cycles down to 150K cycles and very few of those (most are under
100K cycles).
I also don't see perf throttling me down to 1500 samples, it stops around
7000. So I see progress with this patch. :-)
Thanks!
Cheers,
Don
>
> ---
> arch/x86/lib/usercopy.c | 43 +++++++++++++++----------------------------
> arch/x86/mm/fault.c | 43 +++++++++++++++++++++++--------------------
> 2 files changed, 38 insertions(+), 48 deletions(-)
>
> diff --git a/arch/x86/lib/usercopy.c b/arch/x86/lib/usercopy.c
> index 4f74d94c8d97..5465b8613944 100644
> --- a/arch/x86/lib/usercopy.c
> +++ b/arch/x86/lib/usercopy.c
> @@ -11,39 +11,26 @@
> #include <linux/sched.h>
>
> /*
> - * best effort, GUP based copy_from_user() that is NMI-safe
> + * We rely on the nested NMI work to allow atomic faults from the NMI path; the
> + * nested NMI paths are careful to preserve CR2.
> */
> unsigned long
> copy_from_user_nmi(void *to, const void __user *from, unsigned long n)
> {
> - unsigned long offset, addr = (unsigned long)from;
> - unsigned long size, len = 0;
> - struct page *page;
> - void *map;
> - int ret;
> + unsigned long ret;
>
> if (__range_not_ok(from, n, TASK_SIZE))
> - return len;
> -
> - do {
> - ret = __get_user_pages_fast(addr, 1, 0, &page);
> - if (!ret)
> - break;
> -
> - offset = addr & (PAGE_SIZE - 1);
> - size = min(PAGE_SIZE - offset, n - len);
> -
> - map = kmap_atomic(page);
> - memcpy(to, map+offset, size);
> - kunmap_atomic(map);
> - put_page(page);
> -
> - len += size;
> - to += size;
> - addr += size;
> -
> - } while (len < n);
> -
> - return len;
> + return 0;
> +
> + /*
> + * Even though this function is typically called from NMI/IRQ context
> + * disable pagefaults so that its behaviour is consistent even when
> + * called form other contexts.
> + */
> + pagefault_disable();
> + ret = __copy_from_user_inatomic(to, from, n);
> + pagefault_enable();
> +
> + return n - ret;
> }
> EXPORT_SYMBOL_GPL(copy_from_user_nmi);
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 3aaeffcfd67a..506564b13ba7 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -51,7 +51,7 @@ kmmio_fault(struct pt_regs *regs, unsigned long addr)
> return 0;
> }
>
> -static inline int __kprobes notify_page_fault(struct pt_regs *regs)
> +static inline int __kprobes kprobes_fault(struct pt_regs *regs)
> {
> int ret = 0;
>
> @@ -1048,7 +1048,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
> return;
>
> /* kprobes don't want to hook the spurious faults: */
> - if (notify_page_fault(regs))
> + if (kprobes_fault(regs))
> return;
> /*
> * Don't take the mm semaphore here. If we fixup a prefetch
> @@ -1060,23 +1060,8 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
> }
>
> /* kprobes don't want to hook the spurious faults: */
> - if (unlikely(notify_page_fault(regs)))
> + if (unlikely(kprobes_fault(regs)))
> return;
> - /*
> - * It's safe to allow irq's after cr2 has been saved and the
> - * vmalloc fault has been handled.
> - *
> - * User-mode registers count as a user access even for any
> - * potential system fault or CPU buglet:
> - */
> - if (user_mode_vm(regs)) {
> - local_irq_enable();
> - error_code |= PF_USER;
> - flags |= FAULT_FLAG_USER;
> - } else {
> - if (regs->flags & X86_EFLAGS_IF)
> - local_irq_enable();
> - }
>
> if (unlikely(error_code & PF_RSVD))
> pgtable_bad(regs, error_code, address);
> @@ -1088,17 +1073,35 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code)
> }
> }
>
> - perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
> -
> /*
> * If we're in an interrupt, have no user context or are running
> * in an atomic region then we must not take the fault:
> */
> if (unlikely(in_atomic() || !mm)) {
> + if (in_nmi())
> + trace_printk("YIPEE!!!\n");
> bad_area_nosemaphore(regs, error_code, address);
> return;
> }
>
> + /*
> + * It's safe to allow irq's after cr2 has been saved and the
> + * vmalloc fault has been handled.
> + *
> + * User-mode registers count as a user access even for any
> + * potential system fault or CPU buglet:
> + */
> + if (user_mode_vm(regs)) {
> + local_irq_enable();
> + error_code |= PF_USER;
> + flags |= FAULT_FLAG_USER;
> + } else {
> + if (regs->flags & X86_EFLAGS_IF)
> + local_irq_enable();
> + }
> +
> + perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
> +
> if (error_code & PF_WRITE)
> flags |= FAULT_FLAG_WRITE;
>
--
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