Re: [RFC-PATCH 1/2] mm: Add __GFP_NO_LOCKS flag

From: Thomas Gleixner
Date: Wed Aug 12 2020 - 07:44:38 EST

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.

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.