[lkml]   [2014]   [Jun]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [RFC PATCH 4/6] mm, compaction: skip buddy pages by their order in the migrate scanner
On 06/12/2014 02:21 AM, David Rientjes wrote:
> On Wed, 11 Jun 2014, Vlastimil Babka wrote:
>>> I hate to belabor this point, but I think gcc does treat it differently.
>>> If you look at the assembly comparing your patch to if you do
>>> unsigned long freepage_order = ACCESS_ONCE(page_private(page));
>>> instead, then if you enable annotation you'll see that gcc treats the
>>> store as page_x->D.y.private in your patch vs. MEM[(volatile long unsigned
>>> int *)page_x + 48B] with the above.
>> Hm sure you compiled a version that used page_order_unsafe() and not
>> page_order()? Because I do see:
>> MEM[(volatile long unsigned int *)valid_page_114 + 48B];
>> That's gcc 4.8.1, but our gcc guy said he tried 4.5+ and all was like this.
>> And that it would be a gcc bug if not.
>> He also did a test where page_order was called twice in one function and
>> page_order_unsafe twice in another function. page_order() was reduced to a
>> single access in the assembly, page_order_unsafe were two accesses.
> Ok, and I won't continue to push the point.

I'd rather know I'm correct and not just persistent enough :) If you
confirm that your compiler behaves differently, then maybe making
page_order_unsafe a #define instead of inline function would prevent
this issue?

> I think the lockless
> suitable_migration_target() call that looks at page_order() is fine in the
> free scanner since we use it as a racy check, but it might benefit from
> either a comment describing the behavior or a sanity check for
> page_order(page) <= MAX_ORDER as you've done before.

OK, I'll add that.

 \ /
  Last update: 2014-06-12 14:21    [W:0.078 / U:1.884 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site