From nobody Tue Feb 10 23:12:06 2026 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 099F96088F; Mon, 6 Jan 2025 12:02:29 +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=1736164951; cv=none; b=d3ANAvaw56hiEKJTmskEb2gJApPtHYoMcE7PhrTSGIZgIt3Ig577PYjJZf5qR6T7VHRdm9qDkd/Nw0RbvAsIvPwRkGKjajoYB+8l5+8oYksxObLXYZe4DtJTiwFsi+Yep3VjwERjeGB14c+WdCyD29z+k9AMenVPtkbBc3f0epA= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1736164951; c=relaxed/simple; bh=UM6582/aEwucZspCyFUMRY9WesEXLdSRNsOYAXasNLM=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version; b=mqZcpnv5eBs66Dtt00Onor6MCNBvWAjOUavB+AU8fv90O/OX9xBmfpjztmy9iiDKnjDS0TKCnckRI3SlSJSbOlhn3MhEjDW9ERIgrBpV3anag/PlAsRTeZCAd3BzzPR62iWbfChhbTjeSHEK2mwszdQWvoN/63h19r79tyfb9cc= 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 8B5691AC1; Mon, 6 Jan 2025 04:02:57 -0800 (PST) Received: from e128066.arm.com (unknown [10.57.93.133]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id EF2EC3F673; Mon, 6 Jan 2025 04:02:24 -0800 (PST) From: mark.barnett@arm.com To: peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org, irogers@google.com Cc: ben.gainey@arm.com, deepak.surti@arm.com, ak@linux.intel.com, will@kernel.org, james.clark@arm.com, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, adrian.hunter@intel.com, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Mark Barnett Subject: [PATCH v2 1/5] perf: Allow periodic events to alternate between two sample periods Date: Mon, 6 Jan 2025 12:01:52 +0000 Message-Id: <20250106120156.227273-2-mark.barnett@arm.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20250106120156.227273-1-mark.barnett@arm.com> References: <20250106120156.227273-1-mark.barnett@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" From: Ben Gainey This change modifies perf_event_attr to add a second, alternative sample period field, and modifies the core perf overflow handling such that when specified an event will alternate between two sample periods. Currently, perf does not provide a mechanism for decoupling the period over which counters are counted from the period between samples. This is problematic for building a tool to measure per-function metrics derived from a sampled counter group. Ideally such a tool wants a very small sample window in order to correctly attribute the metrics to a given function, but prefers a larger sample period that provides representative coverage without excessive probe effect, triggering throttling, or generating excessive amounts of data. By alternating between a long and short sample_period and subsequently discarding the long samples, tools may decouple the period between samples that the tool cares about from the window of time over which interesting counts are collected. It is expected that typically tools would use this feature with the cycles or instructions events as an approximation for time, but no restrictions are applied to which events this can be applied to. Signed-off-by: Ben Gainey Signed-off-by: Mark Barnett --- include/linux/perf_event.h | 5 +++++ include/uapi/linux/perf_event.h | 3 +++ kernel/events/core.c | 37 ++++++++++++++++++++++++++++++++- 3 files changed, 44 insertions(+), 1 deletion(-) diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h index cb99ec8c9e96..cbb332f4e19c 100644 --- a/include/linux/perf_event.h +++ b/include/linux/perf_event.h @@ -276,6 +276,11 @@ struct hw_perf_event { */ u64 freq_time_stamp; u64 freq_count_stamp; + + /* + * Indicates that the alternative sample period is used + */ + bool using_alt_sample_period; #endif }; =20 diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_even= t.h index 0524d541d4e3..499a8673df8e 100644 --- a/include/uapi/linux/perf_event.h +++ b/include/uapi/linux/perf_event.h @@ -379,6 +379,7 @@ enum perf_event_read_format { #define PERF_ATTR_SIZE_VER6 120 /* add: aux_sample_size */ #define PERF_ATTR_SIZE_VER7 128 /* add: sig_data */ #define PERF_ATTR_SIZE_VER8 136 /* add: config3 */ +#define PERF_ATTR_SIZE_VER9 144 /* add: alt_sample_period */ =20 /* * Hardware event_id to monitor via a performance monitoring event: @@ -531,6 +532,8 @@ struct perf_event_attr { __u64 sig_data; =20 __u64 config3; /* extension of config2 */ + + __u64 alt_sample_period; }; =20 /* diff --git a/kernel/events/core.c b/kernel/events/core.c index 065f9188b44a..7e339d12363a 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -4178,6 +4178,8 @@ static void perf_adjust_period(struct perf_event *eve= nt, u64 nsec, u64 count, bo s64 period, sample_period; s64 delta; =20 + WARN_ON_ONCE(hwc->using_alt_sample_period); + period =3D perf_calculate_period(event, nsec, count); =20 delta =3D (s64)(period - hwc->sample_period); @@ -9850,6 +9852,7 @@ static int __perf_event_overflow(struct perf_event *e= vent, int throttle, struct perf_sample_data *data, struct pt_regs *regs) { + struct hw_perf_event *hwc =3D &event->hw; int events =3D atomic_read(&event->event_limit); int ret =3D 0; =20 @@ -9869,6 +9872,18 @@ static int __perf_event_overflow(struct perf_event *= event, !bpf_overflow_handler(event, data, regs)) goto out; =20 + /* + * Swap the sample period to the alternative period + */ + if (event->attr.alt_sample_period) { + bool using_alt =3D hwc->using_alt_sample_period; + u64 sample_period =3D (using_alt ? event->attr.sample_period + : event->attr.alt_sample_period); + + hwc->sample_period =3D sample_period; + hwc->using_alt_sample_period =3D !using_alt; + } + /* * XXX event_limit might not quite work as expected on inherited * events @@ -12291,9 +12306,19 @@ perf_event_alloc(struct perf_event_attr *attr, int= cpu, if (attr->freq && attr->sample_freq) hwc->sample_period =3D 1; hwc->last_period =3D hwc->sample_period; - local64_set(&hwc->period_left, hwc->sample_period); =20 + if (attr->alt_sample_period) { + hwc->sample_period =3D attr->alt_sample_period; + hwc->using_alt_sample_period =3D true; + } + + /* + * alt_sample_period cannot be used with freq + */ + if (attr->freq && attr->alt_sample_period) + goto err_ns; + /* * We do not support PERF_SAMPLE_READ on inherited events unless * PERF_SAMPLE_TID is also selected, which allows inherited events to @@ -12763,9 +12788,19 @@ SYSCALL_DEFINE5(perf_event_open, if (attr.freq) { if (attr.sample_freq > sysctl_perf_event_sample_rate) return -EINVAL; + if (attr.alt_sample_period) + return -EINVAL; } else { if (attr.sample_period & (1ULL << 63)) return -EINVAL; + if (attr.alt_sample_period) { + if (!attr.sample_period) + return -EINVAL; + if (attr.alt_sample_period & (1ULL << 63)) + return -EINVAL; + if (attr.alt_sample_period =3D=3D attr.sample_period) + attr.alt_sample_period =3D 0; + } } =20 /* Only privileged users can get physical addresses */ --=20 2.43.0