From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 0D2E4C4345F for ; Mon, 22 Apr 2024 13:08:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To: Subject:MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=PUrUnP7j8SyWmrC/V/Q9sRnNWlrFgYsEFTBwLmGHgec=; b=j2OBYgoxYELtgV Oauhgh3FEn1hEkoRdwN9/pNx/W2bTSb3O0e6/XwVNytYT2s0rkJCmfNSG8fviKWsp42ii0sLtTC6k i9N71fkclusS0lBtLIrW8MWtowmN9Dxiq+q2Q3K9+SS+409eo7Zk8l6T8Lwf39O1v/DNVkPyNBEja +6meTbNQPddlvKqBte13vbxnbeyudj/St1xESe6lyqX8OiF2izoZeQxxhUTlt8ejT9wUIAVAT4GV8 P9gfXBLbu/15yt4g0tJdxpIhDyE4iraUp5TPBhVJiNXpQyNw5HlzGGvXOXOgTEklgos/2L35+ngQ0 RUWIMSmOdIZedV+HWTBg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1rytPH-0000000DflD-1oCs; Mon, 22 Apr 2024 13:08:27 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1rytPB-0000000Dfiu-1Auu for linux-arm-kernel@lists.infradead.org; Mon, 22 Apr 2024 13:08:25 +0000 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 57717339; Mon, 22 Apr 2024 06:08:48 -0700 (PDT) Received: from [192.168.1.216] (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1A6723F73F; Mon, 22 Apr 2024 06:08:16 -0700 (PDT) Message-ID: <1dd3692d-dd2c-428f-a7f7-e263d1d5e5c8@arm.com> Date: Mon, 22 Apr 2024 14:08:17 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 2/4] perf: Allow adding fixed random jitter to the alternate sampling period To: Ben Gainey Cc: ak@linux.intel.com, will@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org References: <20240422104929.264241-1-ben.gainey@arm.com> <20240422104929.264241-3-ben.gainey@arm.com> Content-Language: en-US From: James Clark In-Reply-To: <20240422104929.264241-3-ben.gainey@arm.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240422_060821_489861_7C1121E4 X-CRM114-Status: GOOD ( 21.43 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 22/04/2024 11:49, Ben Gainey wrote: > This change modifies the core perf overflow handler, adding some small > random jitter to each sample period whenever an event switches between the > two alternate sample periods. A new flag is added to perf_event_attr to > opt into this behaviour. > > This change follows the discussion in [1], where it is recognized that it > may be possible for certain patterns of execution to end up with biased > results. > > [1] https://lore.kernel.org/linux-perf-users/Zc24eLqZycmIg3d2@tassilo/ > > Signed-off-by: Ben Gainey > --- > include/uapi/linux/perf_event.h | 3 ++- > kernel/events/core.c | 11 ++++++++++- > 2 files changed, 12 insertions(+), 2 deletions(-) > > diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h > index 5c1701d091cf..dd3697a4b300 100644 > --- a/include/uapi/linux/perf_event.h > +++ b/include/uapi/linux/perf_event.h > @@ -461,7 +461,8 @@ struct perf_event_attr { > inherit_thread : 1, /* children only inherit if cloned with CLONE_THREAD */ > remove_on_exec : 1, /* event is removed from task on exec */ > sigtrap : 1, /* send synchronous SIGTRAP on event */ > - __reserved_1 : 26; > + jitter_alternate_period : 1, /* add a limited amount of jitter on each alternate period */ > + __reserved_1 : 25; > > union { > __u32 wakeup_events; /* wakeup every n events */ > diff --git a/kernel/events/core.c b/kernel/events/core.c > index 07f1f931e18e..079ae520e836 100644 > --- a/kernel/events/core.c > +++ b/kernel/events/core.c > @@ -15,6 +15,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -9546,6 +9547,8 @@ static inline bool sample_is_allowed(struct perf_event *event, struct pt_regs *r > return true; > } > > +# define MAX_ALT_SAMPLE_PERIOD_JITTER 16 > + Is 16 enough to make a difference with very large alternate periods? I'm wondering if it's worth making it customisable and instead of adding the boolean option add a 16 bit jitter field. Or the option could still be a boolean but the jitter value is some ratio of the alt sample period, like: get_random_u32_below(max(16, alternative_sample_period >> 4)) > /* > * Generic event overflow handling, sampling. > */ > @@ -9573,7 +9576,10 @@ static int __perf_event_overflow(struct perf_event *event, > if (event->attr.alternative_sample_period) { > bool using_alt = hwc->using_alternative_sample_period; > u64 sample_period = (using_alt ? event->attr.sample_period > - : event->attr.alternative_sample_period); > + : event->attr.alternative_sample_period) > + + (event->attr.jitter_alternate_period > + ? get_random_u32_below(MAX_ALT_SAMPLE_PERIOD_JITTER) > + : 0); > > hwc->sample_period = sample_period; > hwc->using_alternative_sample_period = !using_alt; > @@ -12503,6 +12509,9 @@ SYSCALL_DEFINE5(perf_event_open, > } > } > > + if (attr.jitter_alternate_period && !attr.alternative_sample_period) > + return -EINVAL; > + > /* Only privileged users can get physical addresses */ > if ((attr.sample_type & PERF_SAMPLE_PHYS_ADDR)) { > err = perf_allow_kernel(&attr); _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel