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Date: Fri, 11 Mar 2022 12:09:20 +0100 From: Sebastian Andrzej Siewior <bigeasy@...utronix.de> To: Zqiang <qiang1.zhang@...el.com> Cc: paulmck@...nel.org, frederic@...nel.org, urezki@...il.com, quic_neeraju@...cinc.com, josh@...htriplett.org, juri.lelli@...hat.com, rcu@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH v3] rcu: Only boost rcu reader tasks with lower priority than boost kthreads On 2022-03-11 10:22:26 [+0800], Zqiang wrote: > When RCU_BOOST is enabled, the boost kthreads will boosting readers > who are blocking a given grace period, if the current reader tasks ^ Period. > have a higher priority than boost kthreads(the boost kthreads priority > not always 1, if the kthread_prio is set), This confuses me: - Why does this matter - If it is not RT prio, what is then? Higher or lower? Afaik it is always >= 1. > boosting is useless, skip > current task and select next task to boosting, reduce the time for a > given grace period. So if the task, that is stuck in a rcu_read() section, has a higher priority than the boosting thread then boosting is futile. Understood. Please correct me if I'm wrong but this is intended for !SCHED_OTHER tasks since there shouldn't a be PI chain on boost_mtx so that its default RT priority is boosted above what has been configured. You skip boosting tasks which are itself already boosted due to a PI chain. Once that PI boost is lifted the task may still be in a RCU section. But if I understand you right, your intention is skip boosting tasks with a higher priority and concentrate and those which are in need. This shouldn't make a difference unless the scheduler is able to move the rcu-boosted task to another CPU. Am I right so far? If so this should be part of the commit message (the intention and the result). Also, please add that part with boost_exp_tasks. The comment above boost_mtx is now above boost_exp_tasks with a space so it looks (at least to me) like these two don't belong together. > Suggested-by: Uladzislau Rezki (Sony) <urezki@...il.com> > Signed-off-by: Zqiang <qiang1.zhang@...el.com> Sebastian
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