From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mta0.migadu.com (out-66.mta0.migadu.com [91.218.175.66]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 9B7C14FECE7 for ; Fri, 18 Sep 2026 13:33:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.66 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789738436; cv=none; b=mbGvJK/eOCo6KWx4AGj7yd6Vo0OUSwnCrw6MBIwOU4ICus4MiLaYj8N7IdCmFJa0LpP3FhdZeYafitT04txOHgijgq/sN40+JcXfx36W7wPahqhlI4QA5McMBD22b7kMOklpppChbNpK1J0NsBj4Lfxv1vkcj0tzUc34zAQQkFQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789738436; c=relaxed/simple; bh=QNJzCBnu54BpVshGaNyg8PaNs/5K+yMc5H4BxDJHLDw=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=btZN11ahZQQ9AQ91wnne603QDI6e19n1dw9oRv0eqQE8ewutj04dFhKOi/TkxlGSO43KglV94ZbFoyerGLiCIli6W23D8tjjZ9aqDZWLDlwB6hznMBCCxzCm/HkZSeJHxLS542usJ2MVNebipW9BrG8Kow7Vjb8DZ5f+eeENKvE= 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=B6AqbQjF; arc=none smtp.client-ip=91.218.175.66 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="B6AqbQjF" X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=QNJzCBnu54BpVshGaNyg8PaNs/5K+yMc5H4BxDJHLDw=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1789738431; v=1; x=1790343231; b=B6AqbQjF8D2/PeEcpHq/6ockrVkzrHTgdTOYRrdPl5s0cAd9ZgD1okkTHVhN43Pm+m7Tu9mi KVzvf0cYF6Bsry/aClKAKYEbjiw0y8OdMnqJxYeEoERtYZFJbCOnHQLjIRak5QB+1HsqC7LE24L 5wpNFs/FAxY7+K5HRjVc7jIc= X-Envelope-To: linux-kernel@vger.kernel.org Received: by mta10.migadu.com with ESMTPS id 70f1350159e85615; Fri, 18 Sep 2026 13:33:51 +0000 X-Mizu-Trace-ID: 70f1350159e85615 X-Migadu-Flow: FLOW_OUT From: Usama Arif To: axboe@kernel.dk, cgroups@vger.kernel.org, josef@toxicpanda.com, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, tj@kernel.org Cc: hannes@cmpxchg.org, mkoutny@suse.com, shakeel.butt@linux.dev, riel@surriel.com, Usama Arif Subject: [PATCH] tick/nohz: Avoid unused timekeeping_max_deferment() calls Date: Fri, 18 Sep 2026 06:33:45 -0700 Message-ID: <20260918133345.2823363-1-usama.arif@linux.dev> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit tick_nohz_next_event() limits a CPU's sleep interval to the maximum deferment supported by the current clocksource when that CPU owns the do_timer() duty. If the duty is unassigned, the limit also applies when the CPU's TS_FLAG_DO_TIMER_LAST flag is set. After the early timer checks, the function currently reads the maximum deferment unconditionally. It then replaces the result with KTIME_MAX unless one of the two conditions above applies. timekeeping_max_deferment() performs a seqcount-protected read of the shared timekeeper and follows its clocksource pointer. Check the do_timer state first and avoid this work when the result would be discarded. This leaves the resulting expiry unchanged and reduces accesses to timekeeper data that is modified regularly. On x86-64 this removes 18-20 dynamically executed instructions, including the call, from the common non-owner path when the seqcount does not retry. Signed-off-by: Usama Arif --- v1 -> v2: - Remove the unnecessary comment and delta variable (Frederic Weisbecker). --- kernel/time/tick-sched.c | 23 ++++++++++++----------- 1 file changed, 12 insertions(+), 11 deletions(-) diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index c8f2c4a503b08..a7893a079a83f 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -816,7 +816,7 @@ u64 get_jiffies_update(unsigned long *basej) */ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu) { - u64 basemono, next_tick, delta, expires; + u64 basemono, next_tick, expires; unsigned long basejiff; int tick_cpu; @@ -856,8 +856,7 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu) * If the tick is due in the next period, keep it ticking or * force prod the timer. */ - delta = next_tick - basemono; - if (delta <= (u64)TICK_NSEC) { + if (next_tick - basemono <= (u64)TICK_NSEC) { /* * We've not stopped the tick yet, and there's a timer in the * next period, so no point in stopping it either, bail. @@ -873,17 +872,19 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu) * the sleep time to the timekeeping 'max_deferment' value. * Otherwise we can sleep as long as we want. */ - delta = timekeeping_max_deferment(); tick_cpu = READ_ONCE(tick_do_timer_cpu); if (tick_cpu != cpu && - (tick_cpu != TICK_DO_TIMER_NONE || !tick_sched_flag_test(ts, TS_FLAG_DO_TIMER_LAST))) - delta = KTIME_MAX; - - /* Calculate the next expiry time */ - if (delta < (KTIME_MAX - basemono)) - expires = basemono + delta; - else + (tick_cpu != TICK_DO_TIMER_NONE || !tick_sched_flag_test(ts, TS_FLAG_DO_TIMER_LAST))) { expires = KTIME_MAX; + } else { + expires = timekeeping_max_deferment(); + + /* Calculate the next expiry time */ + if (expires < (KTIME_MAX - basemono)) + expires += basemono; + else + expires = KTIME_MAX; + } ts->timer_expires = min_t(u64, expires, next_tick); -- 2.53.0-Meta