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SubjectRe: [PATCH 6/6] fs-writeback: only allow one inflight and pending !nr_pages flush
On Wed, Sep 20, 2017 at 7:13 AM, Jens Axboe <axboe@kernel.dk> wrote:
> On 09/19/2017 09:10 PM, Amir Goldstein wrote:
>> On Tue, Sep 19, 2017 at 10:53 PM, Jens Axboe <axboe@kernel.dk> wrote:
>>> A few callers pass in nr_pages == 0 when they wakeup the flusher
>>> threads, which means that the flusher should just flush everything
>>> that was currently dirty. If we are tight on memory, we can get
>>> tons of these queued from kswapd/vmscan. This causes (at least)
>>> two problems:
>>>
>>> 1) We consume a ton of memory just allocating writeback work items.
>>> 2) We spend so much time processing these work items, that we
>>> introduce a softlockup in writeback processing.
>>>
>>> Fix this by adding a 'zero_pages' bit to the writeback structure,
>>> and set that when someone queues a nr_pages==0 flusher thread
>>> wakeup. The bit is cleared when we start writeback on that work
>>> item. If the bit is already set when we attempt to queue !nr_pages
>>> writeback, then we simply ignore it.
>>>
>>> This provides us one of full flush in flight, with one pending as
>>> well, and makes for more efficient handling of this type of
>>> writeback.
>>>
>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>>> ---
>>> fs/fs-writeback.c | 30 ++++++++++++++++++++++++++++--
>>> include/linux/backing-dev-defs.h | 1 +
>>> 2 files changed, 29 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>>> index a9a86644cb9f..e0240110b36f 100644
>>> --- a/fs/fs-writeback.c
>>> +++ b/fs/fs-writeback.c
>>> @@ -53,6 +53,7 @@ struct wb_writeback_work {
>>> unsigned int for_background:1;
>>> unsigned int for_sync:1; /* sync(2) WB_SYNC_ALL writeback */
>>> unsigned int auto_free:1; /* free on completion */
>>> + unsigned int zero_pages:1; /* nr_pages == 0 writeback */
>>
>> Suggest: use a name that describes the intention (e.g. WB_everything)
>
> Agree, the name isn't the best. WB_everything isn't great either, though,
> since this isn't an integrity write. WB_start_all would be better,
> I'll make that change.
>
>>> enum wb_reason reason; /* why was writeback initiated? */
>>>
>>> struct list_head list; /* pending work list */
>>> @@ -948,15 +949,25 @@ static void wb_start_writeback(struct bdi_writeback *wb, long nr_pages,
>>> bool range_cyclic, enum wb_reason reason)
>>> {
>>> struct wb_writeback_work *work;
>>> + bool zero_pages = false;
>>>
>>> if (!wb_has_dirty_io(wb))
>>> return;
>>>
>>> /*
>>> - * If someone asked for zero pages, we write out the WORLD
>>> + * If someone asked for zero pages, we write out the WORLD.
>>> + * Places like vmscan and laptop mode want to queue a wakeup to
>>> + * the flusher threads to clean out everything. To avoid potentially
>>> + * having tons of these pending, ensure that we only allow one of
>>> + * them pending and inflight at the time
>>> */
>>> - if (!nr_pages)
>>> + if (!nr_pages) {
>>> + if (test_bit(WB_zero_pages, &wb->state))
>>> + return;
>>> + set_bit(WB_zero_pages, &wb->state);
>>
>> Shouldn't this be test_and_set? not the worst outcome if you have more
>> than one pending work item, but still.
>
> If the frequency of these is high, and they were to trigger the bad
> conditions we saw, then a split test + set is faster as it won't
> keep re-dirtying the same cacheline from multiple locations. It's
> better to leave it a little racy, but faster.
>

Fare enough, but then better change the language of the commit message and
comment above not to claim that there can be only one pending work item.

Amir.

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