+ mm-page_alloc-treat-rt-tasks-similar-to-__gfp_high.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/page_alloc: treat RT tasks similar to __GFP_HIGH
has been added to the -mm mm-unstable branch.  Its filename is
     mm-page_alloc-treat-rt-tasks-similar-to-__gfp_high.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-page_alloc-treat-rt-tasks-similar-to-__gfp_high.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Mel Gorman <mgorman@techsingularity.net>
Subject: mm/page_alloc: treat RT tasks similar to __GFP_HIGH
Date: Fri, 13 Jan 2023 11:12:13 +0000

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.

Link: https://lkml.kernel.org/r/20230113111217.14134-3-mgorman@techsingularity.net
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


--- a/mm/page_alloc.c~mm-page_alloc-treat-rt-tasks-similar-to-__gfp_high
+++ a/mm/page_alloc.c
@@ -4865,7 +4865,7 @@ 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);
 
_

Patches currently in -mm which might be from mgorman@techsingularity.net are

mm-page_alloc-rename-alloc_high-to-alloc_min_reserve.patch
mm-page_alloc-treat-rt-tasks-similar-to-__gfp_high.patch
mm-page_alloc-explicitly-record-high-order-atomic-allocations-in-alloc_flags.patch
mm-page_alloc-explicitly-define-what-alloc-flags-deplete-min-reserves.patch
mm-page_alloc-explicitly-define-how-__gfp_high-non-blocking-allocations-accesses-reserves.patch