From: Anshuman Khandual <anshuman.khandual@arm.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org, will@kernel.org,
catalin.marinas@arm.com, Mark Brown <broonie@kernel.org>,
James Clark <james.clark@arm.com>, Rob Herring <robh@kernel.org>,
Marc Zyngier <maz@kernel.org>,
Suzuki Poulose <suzuki.poulose@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
linux-perf-users@vger.kernel.org
Subject: Re: [PATCH V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE
Date: Fri, 9 Jun 2023 10:00:09 +0530 [thread overview]
Message-ID: <006be03c-43da-6482-6bc4-83fe65dcd706@arm.com> (raw)
In-Reply-To: <ZH3mhorKNo77hsv5@FVFF77S0Q05N>
On 6/5/23 19:13, Mark Rutland wrote:
>> +static u64 branch_type_to_brbcr(int branch_type)
>> +{
>> + u64 brbcr = BRBCR_EL1_DEFAULT_TS;
>> +
>> + /*
>> + * BRBE need not be paused on PMU interrupt while tracing only
>> + * the user space, bcause it will automatically be inside the
>> + * prohibited region. But even after PMU overflow occurs, the
>> + * interrupt could still take much more cycles, before it can
>> + * be taken and by that time BRBE will have been overwritten.
>> + * Let's enable pause on PMU interrupt mechanism even for user
>> + * only traces.
>> + */
>> + brbcr |= BRBCR_EL1_FZP;
> I think this is trying to say that we *should* use FZP when sampling the
> kernel (due to IRQ latency), and *can* safely use it when sampling userspace,
> so it would be good to explain it that way around.
Agreed, following updated comment explains why we should enable FZP
when sampling kernel, otherwise BRBE will capture unwanted records.
It also explains why we should enable FZP even when sampling user
space due to IRQ latency.
/*
* BRBE should be paused on PMU interrupt while tracing kernel
* space to stop capturing further branch records. Otherwise
* interrupt handler branch records might get into the samples
* which is not desired.
*
* BRBE need not be paused on PMU interrupt while tracing only
* the user space, because it will automatically be inside the
* prohibited region. But even after PMU overflow occurs, the
* interrupt could still take much more cycles, before it can
* be taken and by that time BRBE will have been overwritten.
* Hence enable pause on PMU interrupt mechanism even for user
* only traces as well.
*/
brbcr |= BRBCR_EL1_FZP;
>
> It's a bit unfortunate, because where this matters we'll always be losing some
> branches either way, but I guess we don't have much say in the matter.
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next prev parent reply other threads:[~2023-06-09 4:30 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-31 4:04 [PATCH V11 00/10] arm64/perf: Enable branch stack sampling Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 01/10] drivers: perf: arm_pmu: Add new sched_task() callback Anshuman Khandual
2023-06-05 7:26 ` Mark Rutland
2023-05-31 4:04 ` [PATCH V11 02/10] arm64/perf: Add BRBE registers and fields Anshuman Khandual
2023-06-05 7:55 ` Mark Rutland
2023-06-06 4:27 ` Anshuman Khandual
2023-06-13 16:27 ` Mark Rutland
2023-06-14 2:59 ` Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 03/10] arm64/perf: Add branch stack support in struct arm_pmu Anshuman Khandual
2023-06-05 7:58 ` Mark Rutland
2023-06-06 4:47 ` Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 04/10] arm64/perf: Add branch stack support in struct pmu_hw_events Anshuman Khandual
2023-06-05 8:00 ` Mark Rutland
2023-05-31 4:04 ` [PATCH V11 05/10] arm64/perf: Add branch stack support in ARMV8 PMU Anshuman Khandual
2023-06-02 2:33 ` Namhyung Kim
2023-06-05 2:43 ` Anshuman Khandual
2023-06-05 12:05 ` Mark Rutland
2023-06-06 10:34 ` Anshuman Khandual
2023-06-06 10:41 ` Mark Rutland
2023-06-08 10:13 ` Suzuki K Poulose
2023-06-09 4:00 ` Anshuman Khandual
2023-06-09 9:54 ` Suzuki K Poulose
2023-06-09 7:14 ` Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 06/10] arm64/perf: Enable branch stack events via FEAT_BRBE Anshuman Khandual
2023-06-02 1:45 ` Namhyung Kim
2023-06-05 3:00 ` Anshuman Khandual
2023-06-05 13:43 ` Mark Rutland
2023-06-09 4:30 ` Anshuman Khandual [this message]
2023-06-09 12:37 ` Mark Rutland
2023-06-09 4:47 ` Anshuman Khandual
2023-06-09 12:42 ` Mark Rutland
2023-06-09 5:22 ` Anshuman Khandual
2023-06-09 12:47 ` Mark Rutland
2023-06-09 13:15 ` Mark Rutland
2023-06-09 13:34 ` James Clark
2023-05-31 4:04 ` [PATCH V11 07/10] arm64/perf: Add PERF_ATTACH_TASK_DATA to events with has_branch_stack() Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 08/10] arm64/perf: Add struct brbe_regset helper functions Anshuman Khandual
2023-06-02 2:40 ` Namhyung Kim
2023-06-05 3:14 ` Anshuman Khandual
2023-06-05 23:49 ` Namhyung Kim
2023-06-13 17:17 ` Mark Rutland
2023-06-14 5:14 ` Anshuman Khandual
2023-06-14 10:59 ` Mark Rutland
2023-05-31 4:04 ` [PATCH V11 09/10] arm64/perf: Implement branch records save on task sched out Anshuman Khandual
2023-05-31 4:04 ` [PATCH V11 10/10] arm64/perf: Implement branch records save on PMU IRQ Anshuman Khandual
2023-06-09 11:13 ` [PATCH V11 00/10] arm64/perf: Enable branch stack sampling Anshuman Khandual
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