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?
>> > 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.
No.
--
Best Regards,
Huang, Ying
