linux-kernel - Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)

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Date:   Tue, 31 Jan 2023 16:10:19 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>, David Howells <dhowells@...hat.com>
Cc:     John Hubbard <jhubbard@...dia.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Jeff Layton <jlayton@...nel.org>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)

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@...hat.com> 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:

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 815c7c2edf45..a526964b65ce 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -196,8 +196,10 @@ enum node_stat_item {
  	NR_WRITTEN,		/* page writings since bootup */
  	NR_THROTTLED_WRITTEN,	/* NR_WRITTEN while reclaim throttled */
  	NR_KERNEL_MISC_RECLAIMABLE,	/* reclaimable non-slab kernel pages */
+#ifdef CONFIG_DEBUG_VM
  	NR_FOLL_PIN_ACQUIRED,	/* via: pin_user_page(), gup flag: FOLL_PIN */
  	NR_FOLL_PIN_RELEASED,	/* pages returned via unpin_user_page() */
+#endif
  	NR_KERNEL_STACK_KB,	/* measured in KiB */
  #if IS_ENABLED(CONFIG_SHADOW_CALL_STACK)
  	NR_KERNEL_SCS_KB,	/* measured in KiB */
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 1ea6a5ce1c41..5cbd9a1924bf 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1227,8 +1227,10 @@ const char * const vmstat_text[] = {
  	"nr_written",
  	"nr_throttled_written",
  	"nr_kernel_misc_reclaimable",
+#ifdef CONFIG_DEBUG_VM
  	"nr_foll_pin_acquired",
  	"nr_foll_pin_released",
+#endif
  	"nr_kernel_stack",
  #if IS_ENABLED(CONFIG_SHADOW_CALL_STACK)
  	"nr_shadow_call_stack",

-- 
Thanks,

David / dhildenb