On Tue, Sep 22, 2020 at 6:34 AM Mel Gorman <mgorman@xxxxxxx> wrote:
>
> On Mon, Sep 21, 2020 at 09:43:17AM +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. We'd better use
> > inode->i_data.nrpages as the target.
> >
>
> How does that work if the invalidation is for a subset of the file?
>
I realized it as well. There are some solutions to improve it.
Option 1, take the min as the target.
- if (count < (end_index - start_index + 1)) {
+ target = min_t(unsigned long, inode->i_data.nrpages,
+ end_index - start_index + 1);
+ if (count < target) {
lru_add_drain_all();
Option 2, change the prototype of invalidate_mapping_pages and then
check how many pages were skipped.
+ struct invalidate_stat {
+ unsigned long skipped; // how many pages were skipped
+ unsigned long invalidated; // how many pages were invalidated
+};
- unsigned long invalidate_mapping_pages(struct address_space *mapping,
+unsigned long invalidate_mapping_pages(struct address_space *mapping,
struct invalidate_stat *stat,
I prefer option 2.
What do you think ?
--
Thanks
Yafang
