Messages in this thread | | | Date | Thu, 11 May 2023 23:23:58 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 5/7] workqueue: Automatically mark CPU-hogging work items CPU_INTENSIVE |
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On Thu, May 11, 2023 at 08:19:29AM -1000, Tejun Heo wrote: > If a per-cpu work item hogs the CPU, it can prevent other work items from > starting through concurrency management. A per-cpu workqueue which intends > to host such CPU-hogging work items can choose to not participate in > concurrency management by setting %WQ_CPU_INTENSIVE; however, this can be > error-prone and difficult to debug when missed. > > This patch adds an automatic CPU usage based detection. If a > concurrency-managed work item consumes more CPU time than the threshold > (10ms by default) continuously without intervening sleeps, wq_worker_tick() > which is called from scheduler_tick() will detect the condition and > automatically mark it CPU_INTENSIVE. > > The mechanism isn't foolproof: > > * Detection depends on tick hitting the work item. Getting preempted at the > right timings may allow a violating work item to evade detection at least > temporarily.
Right, if you have active tick avoidance in your work items you've got bigger problems :-)
> * nohz_full CPUs may not be running ticks and thus can fail detection.
We do have sched_tick_remote() for the NOHZ_FULL case; it's all a big can of tricky but it might just do -- if you really care ofc.
> * Even when detection is working, the 10ms detection delays can add up if > many CPU-hogging work items are queued at the same time.
HZ=100 assumption there :-) My HZs are bigger 'n yours etc.
> However, in vast majority of cases, this should be able to detect violations > reliably and provide reasonable protection with a small increase in code > complexity. > > If some work items trigger this condition repeatedly, the bigger problem > likely is the CPU being saturated with such per-cpu work items and the > solution would be making them UNBOUND. The next patch will add a debug > mechanism to help spot such cases. > > v3: Switch to use wq_worker_tick() instead of hooking into preemptions as > suggested by Peter. > > v2: Lai pointed out that wq_worker_stopping() also needs to be called from > preemption and rtlock paths and an earlier patch was updated > accordingly. This patch adds a comment describing the risk of infinte > recursions and how they're avoided.
I tend to prefer these changelog-changelogs to go below the --- so that they go away on applying, they're not really relevant when reading the patch in a year's time when trying to figure out wtf this patch did.
> Signed-off-by: Tejun Heo <tj@kernel.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Linus Torvalds <torvalds@linux-foundation.org> > Cc: Lai Jiangshan <jiangshanlai@gmail.com> > ---
Anyway, this seems entirely reasonable.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
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