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Date: Fri, 19 Jun 2020 19:52:32 +0100 From: Valentin Schneider <valentin.schneider@....com> To: Qais Yousef <qais.yousef@....com> Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Patrick Bellasi <patrick.bellasi@...bug.net>, Chris Redpath <chrid.redpath@....com>, Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH 2/2] sched/uclamp: Protect uclamp fast path code with static key On 19/06/20 18:25, Qais Yousef wrote: > On 06/19/20 16:17, Valentin Schneider wrote: > > [...] > >> > But here this is >> > just extra churn. >> > >> > If an imbalance has happend this means either: >> > >> > 1. enqueue/dequeue_task() is imablanced itself >> > 2. uclamp_update_active() calls dec without inc. >> > >> > If 1 happened we have more reasons to be worried about. For 2 the function >> > takes task_rq_lock() and does dec/inc in obvious way. >> > >> >> True. I won't argue over the feasibility of the scenarios we are currently >> aware of, my point was that if they do happen, it's nice to have debug >> helps in the right places as the final breakage can happen much further >> downstream. >> >> FWIW I don't like the diff I suggested at all, but if we can come up with a >> cleverer scheme I think we should do it, as per the above. > > There's the fact as well that this whole thing is to deal with potentially > avoid doing anything that is stricly not necessary in the fast path. > > keep in mind that my patch of introducing the sysctl is not accepted yet > because it introduces such thing, but in that case it's not a debug only > feature. CONFIG_SCHED_DEBUG do get enabled by distros because it exports a lot > of useful info. Sigh, true, but they really shouldn't. The whole point of having SCHED_WARN_ON() is that it's a no-op on !SCHED_DEBUG kernels, which should be any "production" kernel :(
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