On Fri, Sep 25, 2020 at 10:40 PM Mel Gorman <mgorman@xxxxxxx> wrote: > > On Wed, Sep 23, 2020 at 09:33:18PM +0800, Yafang Shao wrote: > > Our users reported that there're some random latency spikes when their RT > > process is running. Finally we found that latency spike is caused by > > FADV_DONTNEED. Which may call lru_add_drain_all() to drain LRU cache on > > remote CPUs, and then waits the per-cpu work to complete. The wait time > > is uncertain, which may be tens millisecond. > > That behavior is unreasonable, because this process is bound to a > > specific CPU and the file is only accessed by itself, IOW, there should > > be no pagecache pages on a per-cpu pagevec of a remote CPU. That > > unreasonable behavior is partially caused by the wrong comparation of the > > number of invalidated pages and the number of the target. For example, > > if (count < (end_index - start_index + 1)) > > The count above is how many pages were invalidated in the local CPU, and > > (end_index - start_index + 1) is how many pages should be invalidated. > > The usage of (end_index - start_index + 1) is incorrect, because they > > are virtual addresses, which may not mapped to pages. Besides that, > > there may be holes between start and end. So we'd better check whether > > there are still pages on per-cpu pagevec after drain the local cpu, and > > then decide whether or not to call lru_add_drain_all(). > > > > After I applied it with a hotfix to our production environment, most of > > the lru_add_drain_all() can be avoided. > > > > Suggested-by: Mel Gorman <mgorman@xxxxxxx> > > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx> > > Cc: Mel Gorman <mgorman@xxxxxxx> > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > > I think that's ok. Does it succeed with the original test case from the > commit that introduced the behaviour and one modified to truncate part > of the mapping? > Yes, I verified the test case in commit 67d46b296a1b and then modified it with truncate. Both works fine. -- Thanks Yafang
