On Wed, Aug 12, 2020 at 01:38:35PM +0200, Thomas Gleixner wrote: > Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes: > > Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes: > >> Michal Hocko <mhocko@xxxxxxxx> writes: > >>> zone->lock should be held for a very limited amount of time. > >> > >> Emphasis on should. free_pcppages_bulk() can hold it for quite some time > >> when a large amount of pages are purged. We surely would have converted > >> it to a raw lock long time ago otherwise. > >> > >> For regular enterprise stuff a few hundred microseconds might qualify as > >> a limited amount of time. For advanced RT applications that's way beyond > >> tolerable.. > > > > Sebastian just tried with zone lock converted to a raw lock and maximum > > latencies go up by a factor of 7 when putting a bit of stress on the > > memory subsytem. Just a regular kernel compile kicks them up by a factor > > of 5. Way out of tolerance. > > > > We'll have a look whether it's solely free_pcppages_bulk() and if so we > > could get away with dropping the lock in the loop. > > So even on !RT and just doing a kernel compile the time spent in > free_pcppages_bulk() is up to 270 usec. > I suspect if you measure the latency of the zone->lock and its contention on any embedded device, i mean not powerful devices like PC, it could be milliseconds. IMHO. > > It's not only the loop which processes a large pile of pages, part of it > is caused by lock contention on zone->lock. Dropping the lock after a > processing a couple of pages does not make it much better if enough CPUs > are contending on the lock. > Initially i have not proposed to convert the lock, because i suspected that from the RT point of view there could be problems. Also, like i mentioned before, the GFP_ATOMIC is not meaningful anymore, that is a bit out of what GFP_ATOMIC stands for. But i see your point about "where is a stop line". That is why i proposed to bail out as later as possible: mm: Add __GFP_NO_LOCKS flag >From the other hand we have been discussing other options, like converting. Just to cover as much as possible :) Thanks Thomas for valuable comments! -- Vlad Rezki
