On Mon, 8 Dec 2014 12:40:52 -0500 Tejun Heo <[email protected]> wrote: > On Thu, Dec 04, 2014 at 10:12:23AM -0500, Tejun Heo wrote: > > From: NeilBrown <[email protected]> > > > > When there is serious memory pressure, all workers in a pool could be > > blocked, and a new thread cannot be created because it requires memory > > allocation. > > > > In this situation a WQ_MEM_RECLAIM workqueue will wake up the > > rescuer thread to do some work. > > > > The rescuer will only handle requests that are already on ->worklist. > > If max_requests is 1, that means it will handle a single request. > > > > The rescuer will be woken again in 100ms to handle another max_requests > > requests. > > > > I've seen a machine (running a 3.0 based "enterprise" kernel) with > > thousands of requests queued for xfslogd, which has a max_requests of > > 1, and is needed for retiring all 'xfs' write requests. When one of > > the worker pools gets into this state, it progresses extremely slowly > > and possibly never recovers (only waited an hour or two). > > > > With this patch we leave a pool_workqueue on mayday list > > until it is clearly no longer in need of assistance. This allows > > all requests to be handled in a timely fashion. > > > > We keep each pool_workqueue on the mayday list until > > need_to_create_worker() is false, and no work for this workqueue is > > found in the pool. > > > > I have tested this in combination with a (hackish) patch which forces > > all work items to be handled by the rescuer thread. In that context > > it significantly improves performance. A similar patch for a 3.0 > > kernel significantly improved performance on a heavy work load. > > > > Thanks to Jan Kara for some design ideas, and to Dongsu Park for > > some comments and testing. > > > > tj: Inverted the lock order between wq_mayday_lock and pool->lock with > > a preceding patch and simplified this patch. Added comment and > > updated changelog accordingly. Dongsu spotted missing get_pwq() > > in the simplified code. > > > > Cc: Dongsu Park <[email protected]> > > Cc: Jan Kara <[email protected]> > > Cc: Lai Jiangshan <[email protected]> > > Signed-off-by: NeilBrown <[email protected]> > > Signed-off-by: Tejun Heo <[email protected]> > > Too late for for-3.18-fixes. Applied the two patches to wq/for-3.19.
I've just run my synthetic test which forces most works to be handled by the rescuer thread. With these patches it works more smoothly than without. And importantly: it still works :-) Thanks, NeilBrown
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