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

[Thomas Gleixner]

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.

Thanks,

        tglx