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Date: Wed, 16 Oct 2013 08:46:49 -0400
From: Don Zickus <dzickus@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: dave.hansen@...ux.intel.com, eranian@...gle.com,
ak@...ux.intel.com, jmario@...hat.com,
linux-kernel@...r.kernel.org, acme@...radead.org, mingo@...nel.org
Subject: Re: [PATCH] perf, x86: Optimize intel_pmu_pebs_fixup_ip()
On Wed, Oct 16, 2013 at 12:57:55PM +0200, Peter Zijlstra wrote:
> A prettier patch below. The main difference is on-demand allocation of
> the scratch buffer.
I'll see if I can sanity test this in the next couple hours.
Further testing yesterday showed that intel_pmu_drain_pebs_nhm still
has long latencies somewhere. With 15 minute reboots, isolation goes
slooow.
Thanks!
Cheers,
Don
>
> ---
> Subject: perf, x86: Optimize intel_pmu_pebs_fixup_ip()
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Tue, 15 Oct 2013 12:14:04 +0200
>
> On Mon, Oct 14, 2013 at 04:35:49PM -0400, Don Zickus wrote:
> > While there are a few places that are causing latencies, for now I focused on
> > the longest one first. It seems to be 'copy_user_from_nmi'
> >
> > intel_pmu_handle_irq ->
> > intel_pmu_drain_pebs_nhm ->
> > __intel_pmu_drain_pebs_nhm ->
> > __intel_pmu_pebs_event ->
> > intel_pmu_pebs_fixup_ip ->
> > copy_from_user_nmi
> >
> > In intel_pmu_pebs_fixup_ip(), if the while-loop goes over 50, the sum of
> > all the copy_from_user_nmi latencies seems to go over 1,000,000 cycles
> > (there are some cases where only 10 iterations are needed to go that high
> > too, but in generall over 50 or so). At this point copy_user_from_nmi
> > seems to account for over 90% of the nmi latency.
>
> So avoid having to call copy_from_user_nmi() for every instruction.
> Since we already limit the max basic block size, we can easily
> pre-allocate a piece of memory to copy the entire thing into in one
> go.
>
> Don reports (for a previous version):
> > Your patch made a huge difference in improvement. The
> > copy_from_user_nmi() no longer hits the million of cycles. I still
> > have a batch of 100,000-300,000 cycles. My longest NMI paths used
> > to be dominated by copy_from_user_nmi, now it is not (I have to dig
> > up the new hot path).
>
> Cc: eranian@...gle.com
> Cc: ak@...ux.intel.com
> Cc: jmario@...hat.com
> Cc: acme@...radead.org
> Cc: dave.hansen@...ux.intel.com
> Reported-by: Don Zickus <dzickus@...hat.com>
> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
> ---
> arch/x86/kernel/cpu/perf_event_intel_ds.c | 48 +++++++++++++++++++++---------
> 1 file changed, 34 insertions(+), 14 deletions(-)
>
> --- a/arch/x86/kernel/cpu/perf_event_intel_ds.c
> +++ b/arch/x86/kernel/cpu/perf_event_intel_ds.c
> @@ -12,6 +12,7 @@
>
> #define BTS_BUFFER_SIZE (PAGE_SIZE << 4)
> #define PEBS_BUFFER_SIZE PAGE_SIZE
> +#define PEBS_FIXUP_SIZE PAGE_SIZE
>
> /*
> * pebs_record_32 for p4 and core not supported
> @@ -228,12 +229,14 @@ void fini_debug_store_on_cpu(int cpu)
> wrmsr_on_cpu(cpu, MSR_IA32_DS_AREA, 0, 0);
> }
>
> +static DEFINE_PER_CPU(void *, insn_buffer);
> +
> static int alloc_pebs_buffer(int cpu)
> {
> struct debug_store *ds = per_cpu(cpu_hw_events, cpu).ds;
> int node = cpu_to_node(cpu);
> int max, thresh = 1; /* always use a single PEBS record */
> - void *buffer;
> + void *buffer, *ibuffer;
>
> if (!x86_pmu.pebs)
> return 0;
> @@ -242,6 +245,15 @@ static int alloc_pebs_buffer(int cpu)
> if (unlikely(!buffer))
> return -ENOMEM;
>
> + if (x86_pmu.intel_cap.pebs_format < 2) {
> + ibuffer = kzalloc_node(PEBS_FIXUP_SIZE, GFP_KERNEL, node);
> + if (!ibuffer) {
> + kfree(buffer);
> + return -ENOMEM;
> + }
> + per_cpu(insn_buffer, cpu) = ibuffer;
> + }
> +
> max = PEBS_BUFFER_SIZE / x86_pmu.pebs_record_size;
>
> ds->pebs_buffer_base = (u64)(unsigned long)buffer;
> @@ -262,6 +274,9 @@ static void release_pebs_buffer(int cpu)
> if (!ds || !x86_pmu.pebs)
> return;
>
> + kfree(per_cpu(insn_buffer, cpu));
> + per_cpu(insn_buffer, cpu) = NULL;
> +
> kfree((void *)(unsigned long)ds->pebs_buffer_base);
> ds->pebs_buffer_base = 0;
> }
> @@ -729,6 +744,7 @@ static int intel_pmu_pebs_fixup_ip(struc
> unsigned long old_to, to = cpuc->lbr_entries[0].to;
> unsigned long ip = regs->ip;
> int is_64bit = 0;
> + void *kaddr;
>
> /*
> * We don't need to fixup if the PEBS assist is fault like
> @@ -752,7 +768,7 @@ static int intel_pmu_pebs_fixup_ip(struc
> * unsigned math, either ip is before the start (impossible) or
> * the basic block is larger than 1 page (sanity)
> */
> - if ((ip - to) > PAGE_SIZE)
> + if ((ip - to) > PEBS_FIXUP_SIZE)
> return 0;
>
> /*
> @@ -763,29 +779,33 @@ static int intel_pmu_pebs_fixup_ip(struc
> return 1;
> }
>
> + if (!kernel_ip(ip)) {
> + int size, bytes;
> + u8 *buf = this_cpu_ptr(insn_buffer);
> +
> + size = ip - to; /* Must fit our buffer, see above */
> + bytes = copy_from_user_nmi(buf, (void __user *)to, size);
> + if (bytes != size)
> + return 0;
> +
> + kaddr = buf;
> + } else {
> + kaddr = (void *)to;
> + }
> +
> do {
> struct insn insn;
> - u8 buf[MAX_INSN_SIZE];
> - void *kaddr;
>
> old_to = to;
> - if (!kernel_ip(ip)) {
> - int bytes, size = MAX_INSN_SIZE;
> -
> - bytes = copy_from_user_nmi(buf, (void __user *)to, size);
> - if (bytes != size)
> - return 0;
> -
> - kaddr = buf;
> - } else
> - kaddr = (void *)to;
>
> #ifdef CONFIG_X86_64
> is_64bit = kernel_ip(to) || !test_thread_flag(TIF_IA32);
> #endif
> insn_init(&insn, kaddr, is_64bit);
> insn_get_length(&insn);
> +
> to += insn.length;
> + kaddr += insn.length;
> } while (to < ip);
>
> if (to == ip) {
--
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