Re: [RFC 2/2] mm: Defer TLB flush by keeping both src and dst folios at migration

[Huang, Ying]

Byungchul Park <byungchul@xxxxxx> writes:

> On Wed, Aug 16, 2023 at 09:01:12AM +0800, Huang, Ying wrote:
>> Byungchul Park <byungchul@xxxxxx> writes:
>> 
>> > On Tue, Aug 15, 2023 at 09:27:26AM +0800, Huang, Ying wrote:
>> >> Byungchul Park <byungchul@xxxxxx> writes:
>> >> 
>> >> > Implementation of CONFIG_MIGRC that stands for 'Migration Read Copy'.
>> >> >
>> >> > We always face the migration overhead at either promotion or demotion,
>> >> > while working with tiered memory e.g. CXL memory and found out TLB
>> >> > shootdown is a quite big one that is needed to get rid of if possible.
>> >> >
>> >> > Fortunately, TLB flush can be defered or even skipped if both source and
>> >> > destination of folios during migration are kept until all TLB flushes
>> >> > required will have been done, of course, only if the target PTE entries
>> >> > have read only permission, more precisely speaking, don't have write
>> >> > permission. Otherwise, no doubt the folio might get messed up.
>> >> >
>> >> > To achieve that:
>> >> >
>> >> >    1. For the folios that have only non-writable TLB entries, prevent
>> >> >       TLB flush by keeping both source and destination of folios during
>> >> >       migration, which will be handled later at a better time.
>> >> >
>> >> >    2. When any non-writable TLB entry changes to writable e.g. through
>> >> >       fault handler, give up CONFIG_MIGRC mechanism so as to perform
>> >> >       TLB flush required right away.
>> >> >
>> >> >    3. TLB flushes can be skipped if all TLB flushes required to free the
>> >> >       duplicated folios have been done by any reason, which doesn't have
>> >> >       to be done from migrations.
>> >> >
>> >> >    4. Adjust watermark check routine, __zone_watermark_ok(), with the
>> >> >       number of duplicated folios because those folios can be freed
>> >> >       and obtained right away through appropreate TLB flushes.
>> >> >
>> >> >    5. Perform TLB flushes and free the duplicated folios pending the
>> >> >       flushes if page allocation routine is in trouble due to memory
>> >> >       pressure, even more aggresively for high order allocation.
>> >> 
>> >> Is the optimization restricted for page migration only?  Can it be used
>> >> for other places?  Like page reclaiming?
>> >
>> > Just to make sure, are you talking about the (5) description? For now,
>> > it's performed at the beginning of __alloc_pages_slowpath(), say, before
>> > page recaiming. Do you think it'd be meaningful to perform it during page
>> > reclaiming? Or do you mean something else?
>> 
>> Not for (5).  TLB needs to be flushed during page reclaiming too.  Can
>> similar method be used to reduce TLB flushing there too?
>
> If you were talking about unmapping for swap while reclaiming, then I
> think yes. The case can also take benefit from CONFIG_MIGRC.

Yes.  Thanks for explanation.

--
Best Regards,
Huang, Ying