Re: [PATCH] mm: fadvise: avoid expensive remote LRU cache draining after FADV_DONTNEED

From: Vlastimil Babka
Date: Mon Dec 12 2016 - 04:21:31 EST

On 12/10/2016 06:26 PM, Johannes Weiner wrote:
When FADV_DONTNEED cannot drop all pages in the range, it observes
that some pages might still be on per-cpu LRU caches after recent
instantiation and so initiates remote calls to all CPUs to flush their
local caches. However, in most cases, the fadvise happens from the
same context that instantiated the pages, and any pre-LRU pages in the
specified range are most likely sitting on the local CPU's LRU cache,
and so in many cases this results in unnecessary remote calls, which,
in a loaded system, can hold up the fadvise() call significantly.

Got any numbers for this part?

Try to avoid the remote call by flushing the local LRU cache before
even attempting to invalidate anything. It's a cheap operation, and
the local LRU cache is the most likely to hold any pre-LRU pages in
the specified fadvise range.

Anyway it looks like things can't be worse after this patch, so...

Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

mm/fadvise.c | 15 ++++++++++++++-
1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/mm/fadvise.c b/mm/fadvise.c
index 6c707bfe02fd..a43013112581 100644
--- a/mm/fadvise.c
+++ b/mm/fadvise.c
@@ -139,7 +139,20 @@ SYSCALL_DEFINE4(fadvise64_64, int, fd, loff_t, offset, loff_t, len, int, advice)

if (end_index >= start_index) {
- unsigned long count = invalidate_mapping_pages(mapping,
+ unsigned long count;
+ /*
+ * It's common to FADV_DONTNEED right after
+ * the read or write that instantiates the
+ * pages, in which case there will be some
+ * sitting on the local LRU cache. Try to
+ * avoid the expensive remote drain and the
+ * second cache tree walk below by flushing
+ * them out right away.
+ */
+ lru_add_drain();
+ count = invalidate_mapping_pages(mapping,
start_index, end_index);