On 1/31/23 8:10 AM, David Hildenbrand wrote:
> On 31.01.23 16:04, Jens Axboe wrote:
>> On 1/31/23 8:02?AM, David Hildenbrand wrote:
>>> On 31.01.23 15:50, Jens Axboe wrote:
>>>> On 1/31/23 6:48?AM, David Hildenbrand wrote:
>>>>> On 31.01.23 14:41, David Howells wrote:
>>>>>> David Hildenbrand <david@xxxxxxxxxx> wrote:
>>>>>>
>>>>>>>>> percpu counters maybe - add them up at the point of viewing?
>>>>>>>> They are percpu, see my last email. But for every 108 changes (on
>>>>>>>> my system), they will do two atomic_long_adds(). So not very
>>>>>>>> useful for anything but low frequency modifications.
>>>>>>>>
>>>>>>>
>>>>>>> Can we just treat the whole acquired/released accounting as a debug mechanism
>>>>>>> to detect missing releases and do it only for debug kernels?
>>>>>>>
>>>>>>>
>>>>>>> The pcpu counter is an s8, so we have to flush on a regular basis and cannot
>>>>>>> really defer it any longer ... but I'm curious if it would be of any help to
>>>>>>> only have a single PINNED counter that goes into both directions (inc/dec on
>>>>>>> pin/release), to reduce the flushing.
>>>>>>>
>>>>>>> Of course, once we pin/release more than ~108 pages in one go or we switch
>>>>>>> CPUs frequently it won't be that much of a help ...
>>>>>>
>>>>>> What are the stats actually used for? Is it just debugging, or do we actually
>>>>>> have users for them (control groups spring to mind)?
>>>>>
>>>>> As it's really just "how many pinning events" vs. "how many unpinning
>>>>> events", I assume it's only for debugging.
>>>>>
>>>>> For example, if you pin the same page twice it would not get accounted
>>>>> as "a single page is pinned".
>>>>
>>>> How about something like the below then? I can send it out as a real
>>>> patch, will run a sanity check on it first but would be surprised if
>>>> this doesn't fix it.
>>>>
>>>>
>>>> diff --git a/mm/gup.c b/mm/gup.c
>>>> index f45a3a5be53a..41abb16286ec 100644
>>>> --- a/mm/gup.c
>>>> +++ b/mm/gup.c
>>>> @@ -168,7 +168,9 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>>>> */
>>>> smp_mb__after_atomic();
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);
>>>> +#endif
>>>> return folio;
>>>> }
>>>> @@ -180,7 +182,9 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
>>>> static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
>>>> {
>>>> if (flags & FOLL_PIN) {
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
>>>> +#endif
>>>> if (folio_test_large(folio))
>>>> atomic_sub(refs, folio_pincount_ptr(folio));
>>>> else
>>>> @@ -236,8 +240,9 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
>>>> } else {
>>>> folio_ref_add(folio, GUP_PIN_COUNTING_BIAS);
>>>> }
>>>> -
>>>> +#ifdef CONFIG_DEBUG_VM
>>>> node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, 1);
>>>> +#endif
>>>> }
>>>> return 0;
>>>>
>>>
>>> We might want to hide the counters completely by defining them only
>>> with CONFIG_DEBUG_VM.
>>
>> Are all of them debug aids only? If so, yes we should just have
>> node_stat_* under CONFIG_DEBUG_VM.
>>
>
> Rather only these 2. Smth like:
Ah gotcha, that makes more sense to me. Will update the patch and
send it out.
--
Jens Axboe
