[2/6] mm/page_alloc: Treat RT tasks similar to __GFP_HIGH

Commit Message

Mel Gorman Jan. 13, 2023, 11:12 a.m. UTC

RT tasks are allowed to dip below the min reserve but ALLOC_HARDER is
typically combined with ALLOC_MIN_RESERVE so RT tasks are a little
unusual. While there is some justification for allowing RT tasks
access to memory reserves, there is a strong chance that a RT task
that is also under memory pressure is at risk of missing deadlines
anyway. Relax how much reserves an RT task can access by treating
it the same as __GFP_HIGH allocations.

Note that in a future kernel release that the RT special casing will be
removed. Hard realtime tasks should be locking down resources in advance
and ensuring enough memory is available. Even a soft-realtime task like
audio or video live decoding which cannot jitter should be allocating both
memory and any disk space required up-front before the recording starts
instead of relying on reserves. At best, reserve access will only delay
the problem by a very short interval.

Signed-off-by: Mel Gorman <[e-mail]>
Acked-by: Vlastimil Babka <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
---
 mm/page_alloc.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 244c1e675dc8..0040b4e00913 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4847,7 +4847,7 @@  gfp_to_alloc_flags(gfp_t gfp_mask)
 		 */
 		alloc_flags &= ~ALLOC_CPUSET;
 	} else if (unlikely(rt_task(current)) && in_task())
-		alloc_flags |= ALLOC_HARDER;
+		alloc_flags |= ALLOC_MIN_RESERVE;
 
 	alloc_flags = gfp_to_alloc_flags_cma(gfp_mask, alloc_flags);