From nobody Sun Nov 24 04:34:29 2024 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 59DE0145B1D; Mon, 22 Apr 2024 10:49:42 +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=1713782983; cv=none; b=KYjZ7uqrqN21weldBTpksc1tauckMahQT8lTyfPFUqpTQxz/mJo3PXUoM7CVp6nGHwQqPTHYGUSXI2zJovkn1rscnM+GsOR45a8DxD10ryPrs33+R49kvEEDS26ZhEW+agd1ECmLhLOBNZWmwaM/LW+59VKEMP/daduEVOOuGZE= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713782983; c=relaxed/simple; bh=JlMo9/yymdi+OtMCepRzEEO4IMsqxHF7a01c0tOVimw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=rVFgFEhCLG55VndRo6AEu9DI16EstZ/XVMSr2oUx4ubk+PGY7wKEm6qgkjBPO9RW/SPIRaEN4LCWkzfozlCxqJ8siV+XQyKnh5VImJ5o/OydxydVaiYx7l6FChyieUvDz2Sb5yrgr5uuwZ/r71iMeKhFIQttWuhUmRv9YTSE2dY= 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 21F4913D5; Mon, 22 Apr 2024 03:50:10 -0700 (PDT) Received: from e126817.cambridge.arm.com (e126817.cambridge.arm.com [10.2.3.5]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id C605B3F7BD; Mon, 22 Apr 2024 03:49:39 -0700 (PDT) From: Ben Gainey To: peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org Cc: ak@linux.intel.com, will@kernel.org, james.clark@arm.com, 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, Ben Gainey Subject: [RFC PATCH v2 2/4] perf: Allow adding fixed random jitter to the alternate sampling period Date: Mon, 22 Apr 2024 11:49:27 +0100 Message-ID: <20240422104929.264241-3-ben.gainey@arm.com> X-Mailer: git-send-email 2.44.0 In-Reply-To: <20240422104929.264241-1-ben.gainey@arm.com> References: <20240422104929.264241-1-ben.gainey@arm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" 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_even= t.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_THR= EAD */ 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; =20 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_even= t *event, struct pt_regs *r return true; } =20 +# define MAX_ALT_SAMPLE_PERIOD_JITTER 16 + /* * 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 =3D hwc->using_alternative_sample_period; u64 sample_period =3D (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); =20 hwc->sample_period =3D sample_period; hwc->using_alternative_sample_period =3D !using_alt; @@ -12503,6 +12509,9 @@ SYSCALL_DEFINE5(perf_event_open, } } =20 + 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 =3D perf_allow_kernel(&attr); --=20 2.44.0