linux-kernel - Re: [PATCH v4 05/38] perf: Add generic exclude

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Message-ID: <aCY_tkjzxknAbEgq@google.com>
Date: Thu, 15 May 2025 12:25:42 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kan Liang <kan.liang@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Mingwei Zhang <mizhang@...gle.com>, 
	Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Paolo Bonzini <pbonzini@...hat.com>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Ian Rogers <irogers@...gle.com>, Adrian Hunter <adrian.hunter@...el.com>, Liang@...gle.com, 
	"H. Peter Anvin" <hpa@...or.com>, linux-perf-users@...r.kernel.org, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, 
	linux-kselftest@...r.kernel.org, Yongwei Ma <yongwei.ma@...el.com>, 
	Xiong Zhang <xiong.y.zhang@...ux.intel.com>, Dapeng Mi <dapeng1.mi@...ux.intel.com>, 
	Jim Mattson <jmattson@...gle.com>, Sandipan Das <sandipan.das@....com>, 
	Zide Chen <zide.chen@...el.com>, Eranian Stephane <eranian@...gle.com>, 
	Shukla Manali <Manali.Shukla@....com>, Nikunj Dadhania <nikunj.dadhania@....com>
Subject: Re: [PATCH v4 05/38] perf: Add generic exclude_guest support

On Thu, May 15, 2025, Kan Liang wrote:
> On 2025-05-14 7:19 p.m., Sean Christopherson wrote:
> >> This naming is confusing on purpose? Pick either guest/host and stick
> >> with it.
> > 
> > +1.  I also think the inner perf_host_{enter,exit}() helpers are superflous.
> > These flows
> > 
> > After a bit of hacking, and with a few spoilers, this is what I ended up with
> > (not anywhere near fully tested).  I like following KVM's kvm_xxx_{load,put}()
> > nomenclature to tie everything together, so I went with "guest" instead of "host"
> > even though the majority of work being down is to shedule out/in host context.
> > 
> > /* When loading a guest's mediated PMU, schedule out all exclude_guest events. */
> > void perf_load_guest_context(unsigned long data)
> > {
> > 	struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
> > 
> > 	lockdep_assert_irqs_disabled();
> > 
> > 	perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> > 
> > 	if (WARN_ON_ONCE(__this_cpu_read(guest_ctx_loaded)))
> > 		goto unlock;
> > 
> > 	perf_ctx_disable(&cpuctx->ctx, EVENT_GUEST);
> > 	ctx_sched_out(&cpuctx->ctx, NULL, EVENT_GUEST);
> > 	perf_ctx_enable(&cpuctx->ctx, EVENT_GUEST);
> > 	if (cpuctx->task_ctx) {
> > 		perf_ctx_disable(cpuctx->task_ctx, EVENT_GUEST);
> > 		task_ctx_sched_out(cpuctx->task_ctx, NULL, EVENT_GUEST);
> > 		perf_ctx_enable(cpuctx->task_ctx, EVENT_GUEST);
> > 	}
> > 
> > 	arch_perf_load_guest_context(data);
> > 
> > 	__this_cpu_write(guest_ctx_loaded, true);
> > 
> > unlock:
> > 	perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
> > }
> > EXPORT_SYMBOL_GPL(perf_load_guest_context);
> > 
> > void perf_put_guest_context(void)
> > {
> > 	struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
> > 
> > 	lockdep_assert_irqs_disabled();
> > 
> > 	perf_ctx_lock(cpuctx, cpuctx->task_ctx);
> > 
> > 	if (WARN_ON_ONCE(!__this_cpu_read(guest_ctx_loaded)))
> > 		goto unlock;
> > 
> > 	arch_perf_put_guest_context();
> 
> It will set the guest_ctx_loaded to false.
> The update_context_time() invoked in the perf_event_sched_in() will not
> get a chance to update the guest time.

The guest_ctx_loaded in arch/x86/events/core.c is a different variable, it just
happens to have the same name.

It's completely gross, but exposing guest_ctx_loaded outside of kernel/events/core.c
didn't seem like a great alternative.  If we wanted to use a single variable,
then the writes in arch_perf_{load,put}_guest_context() can simply go away.