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 D3D8D51474F; Thu, 17 Sep 2026 14:22:21 +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=1789654944; cv=none; b=tBMqaFwe6nd/iQe4aNsd0Y0QYYNDAR8SmZ7862SBLPGirNC+jEr5Ok6kokvpLaM60VqwUEnZrCGhJVpxmRkJrdeWi318kANYiEtrHF0qaRaPajBbk5Hu79LZHVodx5E6cZtN2FOJQjYtVCrq1s5LUk6tWj4xfIVS3QafBfFGZlo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789654944; c=relaxed/simple; bh=qv7Bv+oiJEWlaONHpF0qrpPt4OsKdrgQ3MC2MRmkXR0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UbrU3zNcMT+xsSw3WK/A0b2BejtOYiaRfpJJfkbjJ5M6ST1suR2n143NN9ecaM7/J4odvm5tGU9abQ9uOpkTgn/o6dbud7yliW5VmBDSIty9nROSO0+eRsoPgjakWOM5prmDUN0lopE9SETq1jZRIEgNVL0ipv/fUltzAEe101A= 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; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=ESiJw4AY; 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 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="ESiJw4AY" 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 E11B21476; Thu, 17 Sep 2026 07:22:16 -0700 (PDT) Received: from [10.57.50.144] (unknown [10.57.50.144]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3DD843F86F; Thu, 17 Sep 2026 07:22:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1789654940; bh=qv7Bv+oiJEWlaONHpF0qrpPt4OsKdrgQ3MC2MRmkXR0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ESiJw4AYG+m6Zm4w2RZsLcRgZ4tpG50tgdXQ+DowmSth8SEI2OC+uVfyd3ZFWXAVC m7Dd509835LDdg2Q0KBtXbUC511CVqyNgoPCkvNyerLvPP6ZcGQqrQMNFR8gzhdmGc 500fXWGNkwGbOYA4GR7wP6yN1N/WjEaQ36VR7q9c= Message-ID: Date: Thu, 17 Sep 2026 15:22:15 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/2] sched/fair: Randomize equally shallow slow-path candidates To: Vincent Guittot Cc: Ingo Molnar , Peter Zijlstra , Juri Lelli , Dietmar Eggemann , Steven Rostedt , Valentin Schneider , K Prateek Nayak , Beata Michalska , Elif Topuz , "Rafael J . Wysocki" , Daniel Lezcano , Shubhang Kaushik , Christoph Lameter , linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org References: <20260916100116.701206-1-christian.loehle@arm.com> <20260916100116.701206-3-christian.loehle@arm.com> Content-Language: en-US From: Christian Loehle In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 9/17/26 15:08, Vincent Guittot wrote: > On Wed, 16 Sept 2026 at 12:01, Christian Loehle > wrote: >> >> Picking the first eligible idle CPU leaves a scan-order bias. Concurrent >> slow-path selectors can choose the same CPU before either task is enqueued. >> >> Use reservoir sampling in the tie branch, resetting the candidate count >> when a lower advertised exit latency is found. Use the per-CPU scheduler >> PRNG and reciprocal_scale() to avoid variable division or a second scan. >> >> This reduces deterministic convergence without reserving the chosen CPU. >> >> Signed-off-by: Christian Loehle >> --- >> kernel/sched/fair.c | 11 ++++++++--- >> 1 file changed, 8 insertions(+), 3 deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index ff5793bddc35..6836a8364440 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -24,6 +24,7 @@ >> #include >> #include >> #include >> +#include >> #include >> #include >> #include >> @@ -8459,6 +8460,7 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct * >> { >> unsigned long load, min_load = ULONG_MAX; >> unsigned int min_exit_latency = UINT_MAX; >> + unsigned int nr_candidates = 0; >> int least_loaded_cpu = this_cpu; >> int shallowest_idle_cpu = -1; >> int i; >> @@ -8482,9 +8484,12 @@ sched_balance_find_dst_group_cpu(struct sched_group *group, struct task_struct * >> if (idle && idle->exit_latency < min_exit_latency) { >> min_exit_latency = idle->exit_latency; >> shallowest_idle_cpu = i; >> + nr_candidates = 1; > > You clear the number of candidate when you find a lower exit_latency > but !idle CPUs that have already been checked will be cleared whereas > they still get a chance if they are checked later. That is correct. Is this a problem? (Again, mirroring what upstream currently does, let's say exit_latency(CPU0)=100, exit_latency(CPU2)=1 !idle at CPU3 trumps CPU2 (assuming recent idle_stamp), !idle at CPU1 doesn't trump CPU2, i.e. the inconsistency around !idle is already there?) And I'm assuming we really don't wanna keep a cpumask of !idle CPUs, if anything we may just ignore them completely, given how unlikely it they are to be observed.