Just in case duplicating a mail from the cover-letter thread: It could be done with @cond indeed, that's how it works for now. However, this addresses performance issues only. The problem with wait_event_*() is that, if we have a counter and are trying to wake up tasks after each increment, it would schedule each waiting task O(threshold) times just for it to spuriously check @cond and go back to sleep. All that overhead (memory barriers, registers save/load, accounting, etc) turned out to be enough for some workloads to slow down the system. With this specialisation it still traverses a wait list and makes indirect calls to the checker callback, but the list supposedly is fairly small, so performance there shouldn't be a problem, at least for now. Regarding semantics; It should wake a task when a value passed to wake_up_threshold() is greater or equal then a task's threshold, that is specified individually for each task in wait_threshold_*(). In pseudo code: ``` def wake_up_threshold(n, wait_queue): for waiter in wait_queue: waiter.wake_up_if(n >= waiter.threshold); ``` Any thoughts how to do it better? Ideas are very welcome. BTW, this monster is mostly a copy-paste from wait_event_*(), wait_bit_*(). We could try to extract some common parts from these three, but that's another topic. On 23/09/2019 10:19, Peter Zijlstra wrote: > On Sun, Sep 22, 2019 at 11:08:50AM +0300, Pavel Begunkov (Silence) wrote: >> From: Pavel Begunkov <asml.silence@xxxxxxxxx> >> >> Add wait_threshold -- a custom wait_event derivative, that waits until >> a value is equal to or greater than the specified threshold. > > This is quite insufficient justification for this monster... what exact > semantics do you want? > > Why can't you do this exact same with a slightly more complicated @cond > ? > -- Yours sincerely, Pavel Begunkov
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