On Wed, Apr 28, 2010 at 11:25 PM, Jamie Lokier <jamie@xxxxxxxxxxxxx> wrote: > Changli Gao wrote: >> On Wed, Apr 28, 2010 at 9:21 PM, Jamie Lokier <jamie@xxxxxxxxxxxxx> wrote: >> > Changli Gao wrote: >> >> >> >> fs/eventpoll.c: 1443. >> >> wait.flags |= WQ_FLAG_EXCLUSIVE; >> >> __add_wait_queue(&ep->wq, &wait); >> > >> > The same thing about assumptions applies here. The userspace process >> > may be waiting for an epoll condition to get access to a resource, >> > rather than being a worker thread interchangeable with others. >> >> Oh, the lines above are the current ones. So the assumptions applies >> and works here. > > No, because WQ_FLAG_EXCLUSIVE doesn't have your LIFO semantic at the moment. > > Your patch changes the behaviour of epoll, though I don't know if it > matters. Perhaps all programs which have multiple tasks waiting on > the same epoll fd are "interchangeable worker thread" types anyway :-) > No. You are wrong. I meant epoll implemented LIFO on its own. You should check the code. :) >> > For example, userspace might be using a pipe as a signal-safe lock, or >> > signal-safe multi-token semaphore, and epoll to wait for that pipe. >> > >> > WQ_FLAG_EXCLUSIVE means there is no point waking all tasks, to avoid a >> > pointless thundering herd. It doesn't mean unfairness is ok. >> >> The users should not make any assumption about the waking up sequence, >> neither LIFO nor FIFO. > > Correct, but they should be able to assume non-starvation (eventual > progress) for all waiters. > > It's one of those subtle things, possibly a unixy thing: Non-RT tasks > should always make progress when the competition is just other non-RT > tasks, even if the progress is slow. > > Starvation can spread out beyond the starved process, to cause > priority inversions in other tasks that are waiting on a resource > locked by the starved process. Among other things, that can cause > higher priority tasks, and RT priority tasks, to block permanently. > Very unpleasant. > >> > The LIFO idea _might_ make sense for interchangeable worker-thread >> > situations - including userspace. It would make sense for pipe >> > waiters, socket waiters (especially accept), etc. >> >> Yea, and my following patches are for socket waiters. > > Occasionally unix socketpairs are occasionally used in the above ways too. > > I'm not against your patch, but I worry that starvation is a new > semantic, and it may have a significant effect on something - either > in the kernel, or in userspace which is harder to check. Thanks for your reminding. > > I suspect it's possible to combine LIFO-ish and FIFO-ish queuing to > prevent starvation while getting some of the locality benefit. > Something like add-LIFO and increment a small counter in the next wait > entry, but never add in front of an entry whose counter has reached > MAX_LIFO_WAITERS? :-) > It is a little complex, and I'll keep it simple and improve it when necessary. -- Regards， Changli Gao(xiaosuo@xxxxxxxxx) -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
