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SubjectRe: [PATCH 7/7] fs-writeback: only allow one inflight and pending full flush
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On 09/21/2017 09:05 AM, Christoph Hellwig wrote:
> On Wed, Sep 20, 2017 at 09:33:02AM -0600, Jens Axboe wrote:
>> When someone calls wakeup_flusher_threads() or
>> wakeup_flusher_threads_bdi(), they schedule writeback of all dirty
>> pages in the system (or on that bdi). 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 'start_all' bit to the writeback structure, and
>> set that when someone attempts to flush all dirty page. 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 full flush in flight, with one pending as well,
>> and makes for more efficient handling of this type of writeback.
>>
>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>> Tested-by: Chris Mason <clm@fb.com>
>> Reviewed-by: Jan Kara <jack@suse.cz>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>> fs/fs-writeback.c | 24 ++++++++++++++++++++++++
>> include/linux/backing-dev-defs.h | 1 +
>> 2 files changed, 25 insertions(+)
>>
>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> index 3916ea2484ae..6205319d0c24 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 start_all:1; /* nr_pages == 0 (all) writeback */
>> enum wb_reason reason; /* why was writeback initiated? */
>>
>> struct list_head list; /* pending work list */
>> @@ -953,12 +954,26 @@ static void wb_start_writeback(struct bdi_writeback *wb, bool range_cyclic,
>> return;
>>
>> /*
>> + * All callers of this function want to start writeback of all
>> + * dirty pages. Places like vmscan can call this at a very
>> + * high frequency, causing pointless allocations of tons of
>> + * work items and keeping the flusher threads busy retrieving
>> + * that work. Ensure that we only allow one of them pending and
>> + * inflight at the time
>> + */
>> + if (test_bit(WB_start_all, &wb->state))
>> + return;
>> +
>> + set_bit(WB_start_all, &wb->state);
>
> This should be test_and_set_bit here..

That's on purpose, doesn't matter if we race here, and if we're
being hammered with flusher thread wakeups, then we don't want to
turn that unlocked test into a locked instruction.

> But more importantly once we are not guaranteed that we only have
> a single global wb_writeback_work per bdi_writeback we should just
> embedd that into struct bdi_writeback instead of dynamically
> allocating it.
We could do this as a followup. But right now the logic is that we
can have on started (inflight), and still have one new queued.

--
Jens Axboe

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