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Date: Thu, 22 Dec 2022 14:12:44 -0800 From: Jakub Kicinski <kuba@...nel.org> To: peterz@...radead.org, tglx@...utronix.de Cc: jstultz@...gle.com, edumazet@...gle.com, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Jakub Kicinski <kuba@...nel.org> Subject: [PATCH 3/3] softirq: don't yield if only expedited handlers are pending In networking we try to keep Tx packet queues small, so we limit how many bytes a socket may packetize and queue up. Tx completions (from NAPI) notify the sockets when packets have left the system (NIC Tx completion) and the socket schedules a tasklet to queue the next batch of frames. This leads to a situation where we go thru the softirq loop twice. First round we have pending = NET (from the NIC IRQ/NAPI), and the second iteration has pending = TASKLET (the socket tasklet). On two web workloads I looked at this condition accounts for 10% and 23% of all ksoftirqd wake ups respectively. We run NAPI which wakes some process up, we hit need_resched() and wake up ksoftirqd just to run the TSQ (TCP small queues) tasklet. Tweak the need_resched() condition to be ignored if all pending softIRQs are "non-deferred". The tasklet would run relatively soon, anyway, but once ksoftirqd is woken we're risking stalls. I did not see any negative impact on the latency in an RR test on a loaded machine with this change applied. Signed-off-by: Jakub Kicinski <kuba@...nel.org> --- kernel/softirq.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/softirq.c b/kernel/softirq.c index ad200d386ec1..4ac59ffb0d55 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -601,7 +601,7 @@ asmlinkage __visible void __softirq_entry __do_softirq(void) if (time_is_before_eq_jiffies(end) || !--max_restart) limit = SOFTIRQ_OVERLOAD_TIME; - else if (need_resched()) + else if (need_resched() && pending & ~SOFTIRQ_NOW_MASK) limit = SOFTIRQ_DEFER_TIME; else goto restart; -- 2.38.1
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