Re: [PATCH 1/2] mm/slub: Introduce two counters for the partial objects

[xunlei]

On 2020/8/20 PM9:58, Pekka Enberg wrote:
> Hi Christopher,
> 
> On Tue, Aug 11, 2020 at 3:52 PM Christopher Lameter <cl@xxxxxxxxx> 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@xxxxxxxxx> 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
>