Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag | Date | Wed, 12 Aug 2020 13:38:35 +0200 |
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Thomas Gleixner <tglx@linutronix.de> writes: > Thomas Gleixner <tglx@linutronix.de> writes: >> Michal Hocko <mhocko@suse.com> 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.
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.
Thanks,
tglx
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