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

Uladzislau Rezki
2020-08-12T12:01:33Z

On Wed, Aug 12, 2020 at 01:38:35PM +0200, Thomas Gleixner wrote:
> 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.
>
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