Re: [PATCH v2 2/4] perf/core: Provide a kernel-internal interface to pause perf_event

From: Peter Zijlstra <peterz@infradead.org>
To: Like Xu <like.xu@linux.intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, Jim Mattson <jmattson@google.com>,
	rkrcmar@redhat.com, sean.j.christopherson@intel.com,
	vkuznets@redhat.com, Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	ak@linux.intel.com, wei.w.wang@intel.com, kan.liang@intel.com,
	like.xu@intel.com, ehankland@google.com, arbel.moshe@oracle.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/4] perf/core: Provide a kernel-internal interface to pause perf_event
Date: Mon, 14 Oct 2019 13:51:58 +0200	[thread overview]
Message-ID: <20191014115158.GC2328@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20191013091533.12971-3-like.xu@linux.intel.com>

On Sun, Oct 13, 2019 at 05:15:31PM +0800, Like Xu wrote:
> Exporting perf_event_pause() as an external accessor for kernel users (such
> as KVM) who may do both disable perf_event and read count with just one
> time to hold perf_event_ctx_lock. Also the value could be reset optionally.

> +u64 perf_event_pause(struct perf_event *event, bool reset)
> +{
> +	struct perf_event_context *ctx;
> +	u64 count, enabled, running;
> +
> +	ctx = perf_event_ctx_lock(event);

> +	_perf_event_disable(event);
> +	count = __perf_event_read_value(event, &enabled, &running);
> +	if (reset)
> +		local64_set(&event->count, 0);

This local64_set() already assumes there are no child events, so maybe
write the thing like:

	WARN_ON_ONCE(event->attr.inherit);
	_perf_event_disable(event);
	count = local64_read(&event->count);
	local64_set(&event->count, 0);

> +	perf_event_ctx_unlock(event, ctx);
> +
> +	return count;
> +}
> +EXPORT_SYMBOL_GPL(perf_event_pause);