Messages in this thread Patch in this message | | | Date | Thu, 16 Sep 2021 21:54:18 +0800 | From | Leo Yan <> | Subject | Re: [RFC] perf arm-spe: Track task context switch for cpu-mode events |
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Hi Namhyung,
On Wed, Sep 15, 2021 at 05:17:48PM -0700, Namhyung Kim wrote: > When perf report synthesize events from ARM SPE data, it refers to > current cpu, pid and tid in the machine. But there's no place to set > them in the ARM SPE decoder. I'm seeing all pid/tid is set to -1 and > user symbols are not resolved in the output. > > # perf record -a -e arm_spe_0/ts_enable=1/ sleep 1 > > # perf report -q | head > 8.77% 8.77% :-1 [kernel.kallsyms] [k] format_decode > 7.02% 7.02% :-1 [kernel.kallsyms] [k] seq_printf > 7.02% 7.02% :-1 [unknown] [.] 0x0000ffff9f687c34 > 5.26% 5.26% :-1 [kernel.kallsyms] [k] vsnprintf > 3.51% 3.51% :-1 [kernel.kallsyms] [k] string > 3.51% 3.51% :-1 [unknown] [.] 0x0000ffff9f66ae20 > 3.51% 3.51% :-1 [unknown] [.] 0x0000ffff9f670b3c > 3.51% 3.51% :-1 [unknown] [.] 0x0000ffff9f67c040 > 1.75% 1.75% :-1 [kernel.kallsyms] [k] ___cache_free > 1.75% 1.75% :-1 [kernel.kallsyms] [k] __count_memcg_events > > Like Intel PT, add context switch records to track task info. As ARM > SPE support was added later than PERF_RECORD_SWITCH_CPU_WIDE, I think > we can safely set the attr.context_switch bit and use it.
Thanks for the patch.
Before we had discussion for enabling PID/TID for SPE samples; in the patch set [1], patches 07, 08 set sample's pid/tid based on the Arm SPE context packets. To enable hardware tracing context ID, you also needs to enable kernel config CONFIG_PID_IN_CONTEXTIDR.
At that time, there have a concern is the hardware context ID might introduce confusion for non-root namespace.
We also considered to use PERF_RECORD_SWITCH_CPU_WIDE event for setting pid/tid, the Intel PT implementation uses two things to set sample's pid/tid: one is PERF_RECORD_SWITCH_CPU_WIDE event and another is to detect the branch instruction is the symbol "__switch_to". Since the trace event PERF_RECORD_SWITCH_CPU_WIDE is coarse, so it only uses the new pid/tid after the branch instruction for "__switch_to". Arm SPE is 'statistical', thus it cannot promise the trace data must contain the branch instruction for "__switch_to", please see details [2].
I think the feasible way is to use CONTEXTIDR to trace PID/TID _only_ for root namespace, and the perf tool uses context packet to set pid/tid for samples. So except we need patches 07 and 08, we also need a change in Arm SPE driver as below:
diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c index d44bcc29d99c..2553d53d3772 100644 --- a/drivers/perf/arm_spe_pmu.c +++ b/drivers/perf/arm_spe_pmu.c @@ -272,7 +272,9 @@ static u64 arm_spe_event_to_pmscr(struct perf_event *event) if (!attr->exclude_kernel) reg |= BIT(SYS_PMSCR_EL1_E1SPE_SHIFT); - if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && perfmon_capable()) + /* Only enable context ID tracing for root namespace */ + if (IS_ENABLED(CONFIG_PID_IN_CONTEXTIDR) && perfmon_capable() && + (task_active_pid_ns(current) == &init_pid_ns)) reg |= BIT(SYS_PMSCR_EL1_CX_SHIFT); return reg; Could you confirm if this works for you? If it's okay for you, I will sync with James for upstreaming the changes.
Thanks, Leo
[1] https://lore.kernel.org/lkml/20210119144658.793-8-james.clark@arm.com/ [2] https://lore.kernel.org/lkml/20210204102734.GA4737@leoy-ThinkPad-X240s/
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