Re: [PATCH 2/2] mm/pageblock: remove false sharing in pageblock_flags

[Alex Shi]

在 2020/8/16 下午12:17, Matthew Wilcox 写道:
> On Sun, Aug 16, 2020 at 11:47:57AM +0800, Alex Shi wrote:
>> Current pageblock_flags is only 4 bits, so it has to share a char size
>> in cmpxchg when get set, the false sharing cause perf drop.
>>
>> If we incrase the bits up to 8, false sharing would gone in cmpxchg. and
>> the only cost is half char per pageblock, which is half char per 128MB
>> on x86, 4 chars in 1 GB.
> 
> I don't believe this patch has that effect, mostly because it still does
> cmpxchg() on words instead of bytes.

Hi Matthew,

Thank a lot for comments!

Sorry, I must overlook sth, would you like point out why the cmpxchg is still
on words after patch 1 applied?


> 
> But which functions would benefit?  It seems to me this cmpxchg() is
> only called from the set_pageblock_migratetype() morass of functions,
> none of which are called in hot paths as far as I can make out.
> 
> So are you just reasoning by analogy with the previous patch where you
> have measured a performance improvement, or did you send the wrong patch,
> or did I overlook a hot path that calls one of the pageblock migration
> functions?
> 

Uh, I am reading compaction.c and found the following commit introduced 
test_and_set_skip under a lock. It looks like the pagelock_flags setting
has false sharing in cmpxchg. but I have no valid data on this yet.

Thanks
Alex

e380bebe4771548  mm, compaction: keep migration source private to a single compaction instance

                if (!locked) {
                        locked = compact_trylock_irqsave(zone_lru_lock(zone),
                                                                &flags, cc);
-                       if (!locked)
+
+                       /* Allow future scanning if the lock is contended */
+                       if (!locked) {
+                               clear_pageblock_skip(page);
                                break;
+                       }
+
+                       /* Try get exclusive access under lock */
+                       if (!skip_updated) {
+                               skip_updated = true;
+                               if (test_and_set_skip(cc, page, low_pfn))
+                                       goto isolate_abort;
+                       }