lkml.org 
[lkml]   [2020]   [Aug]   [24]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects
From
Date
On 2020/8/20 PM9:58, Pekka Enberg wrote:
> Hi Christopher,
>
> On Tue, Aug 11, 2020 at 3:52 PM Christopher Lameter <cl@linux.com> wrote:
>>
>> On Fri, 7 Aug 2020, Pekka Enberg wrote:
>>
>>> Why do you consider this to be a fast path? This is all partial list
>>> accounting when we allocate/deallocate a slab, no? Just like
>>> ___slab_alloc() says, I assumed this to be the slow path... What am I
>>> missing?
>>
>> I thought these were per object counters? If you just want to count the
>> number of slabs then you do not need the lock at all. We already have a
>> counter for the number of slabs.
>
> The patch attempts to speed up count_partial(), which holds on to the
> "n->list_lock" (with IRQs off) for the whole duration it takes to walk
> the partial slab list:
>
> spin_lock_irqsave(&n->list_lock, flags);
> list_for_each_entry(page, &n->partial, slab_list)
> x += get_count(page);
> spin_unlock_irqrestore(&n->list_lock, flags);
>
> It's counting the number of *objects*, but the counters are only
> updated in bulk when we add/remove a slab to/from the partial list.
> The counter updates are therefore *not* in the fast-path AFAICT.
>
> Xunlei, please correct me if I'm reading your patches wrong.

Yes, it's all in slow-path.

>
> On Tue, Aug 11, 2020 at 3:52 PM Christopher Lameter <cl@linux.com> wrote:
>>> No objections to alternative fixes, of course, but wrapping the
>>> counters under CONFIG_DEBUG seems like just hiding the actual issue...
>>
>> CONFIG_DEBUG is on by default. It just compiles in the debug code and
>> disables it so we can enable it with a kernel boot option. This is because
>> we have had numerous issues in the past with "production" kernels that
>> could not be recompiled with debug options. So just running the prod
>> kernel with another option will allow you to find hard to debug issues in
>> a full scale producton deployment with potentially proprietary modules
>> etc.
>
> Yeah, it's been too long since I last looked at the code and did not
> realize even count_partial() is wrapped in CONFIG_DEBUG. So by all

Besides CONFIG_DEBUG, count_partial() is also wrapped in CONFIG_SYSFS.

> means, let's also wrap the counters with that too.
>
> - Pekka
>

\
 
 \ /
  Last update: 2020-08-24 12:00    [W:0.121 / U:0.128 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site