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

[Byungchul Park]

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?

> > The measurement result:
> >
> >    Architecture - x86_64
> >    QEMU - kvm enabled, host cpu, 2nodes((4cpus, 2GB)+(cpuless, 6GB))
> >    Linux Kernel - v6.4, numa balancing tiering on, demotion enabled
> >    Benchmark - XSBench with no parameter changed
> >
> >    run 'perf stat' using events:
> >    (FYI, process wide result ~= system wide result(-a option))
> >       1) itlb.itlb_flush
> >       2) tlb_flush.dtlb_thread
> >       3) tlb_flush.stlb_any
> >
> >    run 'cat /proc/vmstat' and pick up:
> >       1) pgdemote_kswapd
> >       2) numa_pages_migrated
> >       3) pgmigrate_success
> >       4) nr_tlb_remote_flush
> >       5) nr_tlb_remote_flush_received
> >       6) nr_tlb_local_flush_all
> >       7) nr_tlb_local_flush_one
> >
> >    BEFORE - mainline v6.4
> >    ==========================================
> >
> >    $ perf stat -e itlb.itlb_flush,tlb_flush.dtlb_thread,tlb_flush.stlb_any ./XSBench
> >
> >    Performance counter stats for './XSBench':
> >
> >       426856       itlb.itlb_flush
> >       6900414      tlb_flush.dtlb_thread
> >       7303137      tlb_flush.stlb_any
> >
> >    33.500486566 seconds time elapsed
> >    92.852128000 seconds user
> >    10.526718000 seconds sys
> >
> >    $ cat /proc/vmstat
> >
> >    ...
> >    pgdemote_kswapd 1052596
> >    numa_pages_migrated 1052359
> >    pgmigrate_success 2161846
> >    nr_tlb_remote_flush 72370
> >    nr_tlb_remote_flush_received 213711
> >    nr_tlb_local_flush_all 3385
> >    nr_tlb_local_flush_one 198679
> >    ...
> >
> >    AFTER - mainline v6.4 + CONFIG_MIGRC
> >    ==========================================
> >
> >    $ perf stat -e itlb.itlb_flush,tlb_flush.dtlb_thread,tlb_flush.stlb_any ./XSBench
> >
> >    Performance counter stats for './XSBench':
> >
> >       179537       itlb.itlb_flush
> >       6131135      tlb_flush.dtlb_thread
> >       6920979      tlb_flush.stlb_any
> 
> It appears that the number of "itlb.itlb_flush" changes much, but not
> for other 2 events.  Because the text segment of the executable file is
> mapped as read-only?  And most other pages are mapped read-write?

Yes, for this benchmarch, XSBench. I didn't noticed that until checking
it using perf event either.

> >    30.396700625 seconds time elapsed
> >    80.331252000 seconds user
> >    10.303761000 seconds sys
> >
> >    $ cat /proc/vmstat
> >
> >    ...
> >    pgdemote_kswapd 1044602
> >    numa_pages_migrated 1044202
> >    pgmigrate_success 2157808
> >    nr_tlb_remote_flush 30453
> >    nr_tlb_remote_flush_received 88840
> >    nr_tlb_local_flush_all 3039
> >    nr_tlb_local_flush_one 198875
> >    ...
> >
> > Signed-off-by: Byungchul Park <byungchul@xxxxxx>

[...]

> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 306a3d1a0fa6..3be66d3eabd2 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -228,6 +228,10 @@ struct page {
> >  #ifdef LAST_CPUPID_NOT_IN_PAGE_FLAGS
> >  	int _last_cpupid;
> >  #endif
> > +#ifdef CONFIG_MIGRC
> > +	struct llist_node migrc_node;
> > +	unsigned int migrc_state;
> > +#endif
> 
> We cannot enlarge "struct page".

This is what I worried about. Do you have a better idea? I don't think
they fit onto page_ext or something.

	Byungchul