From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-189.mta0.migadu.com (out-189.mta0.migadu.com [91.218.175.189]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 8AAD8330B29 for ; Wed, 17 Jun 2026 17:52:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.189 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781718762; cv=none; b=h44lqYwpPgF4RaLqPilB8e97iHezgLJg1lHCf6imOpFDfRuD0+LU+IfKUMw5IRyPX/DbP/2iTfdx/LrQLqA6XsW/cAxBBBAayybK+tuYtFlZqGsH3g3LaM7Chg1I75Rjf2yuKtKXcWKWCbKeFFpTIMEfkaS2asrLO1GqRXGf+Nw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1781718762; c=relaxed/simple; bh=SWQwp/2JYKjKhfHP48hWldsrjys3Lp2RuI3i7F1np7o=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=RUDKKuYQTEvZsX2uJ+S6r5APxvrVRNEoYFcYJAGoMMV/c2QGY5pMcKEhLHoaPmP+nymQi4v/QgU071yZQiqyS+z2olBPxaQ1nCutGN4zmElfUO6xw9tTisoX/CbYn6M/6LFmtdlyc8V03kVisaaY8CLIMSIuBVtS33DhArxElfU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=jHY73IPw; arc=none smtp.client-ip=91.218.175.189 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="jHY73IPw" X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1781718758; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=iT3/EFWjQcFNWU8AZui0piojLffybLEkWSqmoNZTAr0=; b=jHY73IPwI4mKEO8XYiYFNaf4NQx8nholKnr9xjEOM86sr3tHDjl4Cc6Q2SdtUQDAM5Jpl3 7oaiCztd70GO/zjNtSKRs1+Izogub3wQanuozLnLkMKGYlLGWIfEtXmi1GMjY9Lrfaa+9c 1K//wasyi0osGPvoU4Ctsvav6FpBZw4= From: Usama Arif To: Andrew Morton , david@kernel.org, linux-mm@kvack.org, bsegall@google.com, dietmar.eggemann@arm.com, hannes@cmpxchg.org, juri.lelli@redhat.com, kprateek.nayak@amd.com, linux-kernel@vger.kernel.org, mgorman@suse.de, mingo@redhat.com, peterz@infradead.org, rostedt@goodmis.org, surenb@google.com, vincent.guittot@linaro.org, vschneid@redhat.com Cc: shakeel.butt@linux.dev, riel@surriel.com, kernel-team@meta.com, Usama Arif Subject: [PATCH 1/1] sched/psi: skip irqtime accounting when no new irq time has elapsed Date: Wed, 17 Jun 2026 10:50:06 -0700 Message-ID: <20260617175219.2494857-2-usama.arif@linux.dev> In-Reply-To: <20260617175219.2494857-1-usama.arif@linux.dev> References: <20260617175219.2494857-1-usama.arif@linux.dev> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT psi_account_irqtime() reads irq_time_read() into a per-rq cumulative counter and only bails out when the delta vs. the previously accounted amount is negative. A delta of exactly zero is treated as "do the work": psi_write_begin() is taken, cpu_clock(cpu) is read (which on x86 ends up in native_sched_clock() / rdtsc) and the cgroup ancestor chain is walked to add zero to every group's PSI_IRQ_FULL bucket. The zero-delta case is common in practice -- it fires every time a context switch crosses a PSI group boundary on a CPU that hasn't serviced an interrupt between the two switches. Measured on a 176-thread AMD EPYC 9D64 server running a compute intensive production workload, instrumented with bpftrace over a 30s window (irq_time_read() read directly from the per-CPU cpu_irqtime so that delta == 0 and delta < 0 could be separated): @total 17,229,311 (100.0%) @ret_curr_swapper 7,864,195 ( 45.6%) curr->pid == 0 @ret_samegrp 323,299 ( 1.9%) same cgroup as prev @reached_delta 9,041,817 ( 52.5%) @delta_positive 6,358,192 ( 36.9%) real work @delta_zero 2,683,625 ( 15.6%) work wasted (this patch) @delta_negative (0) ( 0.0%) monotonic clock So 15.6 % of all psi_account_irqtime() calls - and 29.7 % of the calls that get past the early returns - hit the delta == 0 case; delta < 0 did not occur once in the 30 s window. Under the current code each of those ~89 k calls per second performs the full seqcount write + cpu_clock() read + cgroup-chain walk just to add 0 to every group's PSI_IRQ_FULL counter. Extend the early-return to also cover delta == 0. rq->psi_irq_time does not need updating in that case (it would store the same value back) and no PSI bucket would change. The existing behaviour for delta > 0 is untouched. Signed-off-by: Usama Arif --- kernel/sched/psi.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c index d9c9d9480a45..848955f8893d 100644 --- a/kernel/sched/psi.c +++ b/kernel/sched/psi.c @@ -1023,7 +1023,7 @@ void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_st irq = irq_time_read(cpu); delta = (s64)(irq - rq->psi_irq_time); - if (delta < 0) + if (delta <= 0) return; rq->psi_irq_time = irq; -- 2.53.0-Meta