linux-kernel - Re: [Patch v3 12/22] perf/x86/intel: Update dyn_constranit base on PEBS event precise level

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Message-ID: <607b1f13-1d5d-4ea7-b0ab-f4c7f4fa319b@linux.intel.com>
Date: Tue, 15 Apr 2025 12:31:03 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>,
 Dapeng Mi <dapeng1.mi@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo
 <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>,
 Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Andi Kleen <ak@...ux.intel.com>, Eranian Stephane <eranian@...gle.com>,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 Dapeng Mi <dapeng1.mi@...el.com>
Subject: Re: [Patch v3 12/22] perf/x86/intel: Update dyn_constranit base on
 PEBS event precise level



On 2025-04-15 9:53 a.m., Peter Zijlstra wrote:
> On Tue, Apr 15, 2025 at 11:44:18AM +0000, Dapeng Mi wrote:
>> arch-PEBS provides CPUIDs to enumerate which counters support PEBS
>> sampling and precise distribution PEBS sampling. Thus PEBS constraints
>> should be dynamically configured base on these counter and precise
>> distribution bitmap instead of defining them statically.
>>
>> Update event dyn_constraint base on PEBS event precise level.
> 
> What if any constraints are there on this? 

Do you mean the static constraints defined in the
event_constraints/pebs_constraints?

> CPUID is virt host
> controlled, right, so these could be the most horrible masks ever.
>

Yes, it could be changed by VMM. A sanity check should be required if
abad mask is given.

> This can land us in EVENT_CONSTRAINT_OVERLAP territory, no?The dyn_constraint is a supplement of the static constraints. It doesn't
overwrite the static constraints.

In the intel_get_event_constraints(), perf always gets the static
constraints first. If the dyn_constraint is defined, it gets the common
mask of the static constraints and the dynamic constraints. All
constraint rules will be complied.

	if (event->hw.dyn_constraint != ~0ULL) {
		c2 = dyn_constraint(cpuc, c2, idx);
		c2->idxmsk64 &= event->hw.dyn_constraint;
		c2->weight = hweight64(c2->idxmsk64);
	}

Thanks,
Kan