From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 9643721A457; Mon, 9 Jun 2025 18:36:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749494172; cv=none; b=bJvc5KORlyickVvmCcFbfkD/VDelnVfXeLjquA9imvHjYx6v7PBngTxqGbNm/0VKDtp163m1Kx8VFzI6sWXrlGxYlZ+esDKYHTxtGferHffYN9h9tIl+KxXa8VWqGkuJujAbD6AmWjvbrs3MjUfg8fHEIZgZHn6OLiWf8A4I2iw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749494172; c=relaxed/simple; bh=44A7002utCBEPUuoju0t5n9X09SGYJUJuABq0b/70vc=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=UuRjcNrdP7mwD++UavJvg5jkcDzhBl9TfcMMV4autN/rDO/QQogch7KuafcXfNWQFgA22mtouY88Ob7IGOzNEITzyAauoNXTtlM4OE5ykWdYeh+2IYOjiI+/Mt9kfB78wI99FN6fIhCKp4r5DDYME8k8/rMGcV4cbiPxcZktD1c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A5F40150C; Mon, 9 Jun 2025 11:35:50 -0700 (PDT) Received: from localhost (e132581.arm.com [10.1.196.87]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 407503F673; Mon, 9 Jun 2025 11:36:09 -0700 (PDT) Date: Mon, 9 Jun 2025 19:36:04 +0100 From: Leo Yan To: "Liang, Kan" Cc: peterz@infradead.org, mingo@redhat.com, namhyung@kernel.org, irogers@google.com, mark.rutland@arm.com, linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, eranian@google.com, ctshao@google.com, tmricht@linux.ibm.com, Aishwarya TCV , Alexei Starovoitov , Venkat Rao Bagalkote , Vince Weaver Subject: Re: [PATCH V4] perf: Fix the throttle error of some clock events Message-ID: <20250609183604.GP8020@e132581.arm.com> References: <20250606192546.915765-1-kan.liang@linux.intel.com> <20250609123415.GJ8020@e132581.arm.com> Precedence: bulk X-Mailing-List: linux-perf-users@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jun 09, 2025 at 09:48:12AM -0400, Liang, Kan wrote: [...] > >> Move event->hw.interrupts = MAX_INTERRUPTS before the stop(). It makes > >> the order the same as perf_event_unthrottle(). Except the patch, no one > >> checks the hw.interrupts in the stop(). There is no impact from the > >> order change. > >> > >> When stops in the throttle, the event should not be updated, > >> stop(event, 0). > > > > I am confused for this conclusion. When a CPU or task clock event is > > stopped by throttling, should it also be updated? Otherwise, we will > > lose accouting for the period prior to the throttling. > > > > I saw you exchanged with Alexei for a soft lockup issue, the reply [1] > > shows that skipping event update on throttling does not help to > > resolve the lockup issue. > > > > Could you elaberate why we don't need to update clock events when > > throttling? > > > > This is to follow the existing behavior before the throttling fix*. > > When throttling is triggered, the stop(event, 0); will be invoked. > As my understanding, it's because the period is not changed with > throttling. So we don't need to update the period. > But if the period is changed, the update is required. You may find an > example in the perf_adjust_freq_unthr_events(). In the freq mode, > stop(event, PERF_EF_UPDATE) is actually invoked for the triggered event. > For the clock event, the existing behavior before the throttling fix* is > not to invoke the stop() in throttling. It relies on the > HRTIMER_NORESTART instead. My previous throttling fix changes the > behavior. It invokes both stop() and HRTIMER_NORESTART. Now, this patch > change the behavior back. Actually, the "event->count" has been updated in perf_swevent_hrtimer(), this is why this patch does not cause big deviation if skip updating count in the ->stop() callback: perf_swevent_hrtimer() ` event->pmu->read(event); => Update count ` __perf_event_overflow() ` perf_event_throttle() ` event->pmu->stop(event, 0) / cpu_clock_event_stop() ` perf_swevent_cancel_hrtimer() => Skip to cancel timer ` task_clock_event_update() => Skip to update count ` return HRTIMER_NORESTART; => Stop timer It is a bit urgly that we check the throttling separately in two places: one is in perf_swevent_cancel_hrtime() for skipping cancel timer, and then we skip updating event count in cpu_clock_event_stop(). One solution is it would be fine to update count in ->stop() callback for the throttling. This should not cause any issue (though it is a bit redundant that the count is updated twice). Or even more clear, we can define a flag PERF_EF_THROTTLING: #define PERF_EF_THROTTLING 0x20 event->pmu->stop(event, PERF_EF_THROTTLING); cpu_clock_event_stop(struct perf_event *event, int flags) { if (flags == PERF_EF_THROTTLING) return; .... } This might need to do a wider checking to ensure this new flags will not cause any issues. Thanks, Leo