+ this_cpu_cmpxchg-arm64-switch-this_cpu_cmpxchg-to-locked-add-_local-function.patch added to mm-unstable branch

* + this_cpu_cmpxchg-arm64-switch-this_cpu_cmpxchg-to-locked-add-_local-function.patch added to mm-unstable branch
@ 2023-03-13 18:07 Andrew Morton
  0 siblings, 0 replies; 2+ messages in thread
From: Andrew Morton @ 2023-03-13 18:07 UTC (permalink / raw)
  To: mm-commits, willy, vbabka, thomas.hellstrom, peterx, mhocko,
	lstoakes, linux, kirill.shutemov, jgg, hca, frederic,
	dan.j.williams, cl, christian.koenig, chenhuacai, atomlin,
	mtosatti, akpm

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The patch titled
     Subject: this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function
has been added to the -mm mm-unstable branch.  Its filename is
     this_cpu_cmpxchg-arm64-switch-this_cpu_cmpxchg-to-locked-add-_local-function.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/this_cpu_cmpxchg-arm64-switch-this_cpu_cmpxchg-to-locked-add-_local-function.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: Marcelo Tosatti <mtosatti@redhat.com>
Subject: this_cpu_cmpxchg: ARM64: switch this_cpu_cmpxchg to locked, add _local function
Date: Mon, 13 Mar 2023 13:25:08 -0300

Patch series "fold per-CPU vmstats remotely", v5.

This patch series addresses the following two problems:

    1. A customer provided some evidence which indicates that
       the idle tick was stopped; albeit, CPU-specific vmstat
       counters still remained populated.

       Thus one can only assume quiet_vmstat() was not
       invoked on return to the idle loop. If I understand
       correctly, I suspect this divergence might erroneously
       prevent a reclaim attempt by kswapd. If the number of
       zone specific free pages are below their per-cpu drift
       value then zone_page_state_snapshot() is used to
       compute a more accurate view of the aforementioned
       statistic.  Thus any task blocked on the NUMA node
       specific pfmemalloc_wait queue will be unable to make
       significant progress via direct reclaim unless it is
       killed after being woken up by kswapd
       (see throttle_direct_reclaim())

    2. With a SCHED_FIFO task that busy loops on a given CPU,
       and kworker for that CPU at SCHED_OTHER priority,
       queuing work to sync per-vmstats will either cause that
       work to never execute, or stalld (i.e. stall daemon)
       boosts kworker priority which causes a latency
       violation

By having vmstat_shepherd flush the per-CPU counters to the global
counters from remote CPUs.

This is done using cmpxchg to manipulate the counters, both CPU locally
(via the account functions), and remotely (via cpu_vm_stats_fold).

Thanks to Aaron Tomlin for diagnosing issue 1 and writing the initial
patch series.


This patch (of 5):

Goal is to have vmstat_shepherd to transfer from per-CPU counters to
global counters remotely.  For this, an atomic this_cpu_cmpxchg is
necessary.

Following the kernel convention for cmpxchg/cmpxchg_local, change ARM's
this_cpu_cmpxchg_ helpers to be atomic, and add this_cpu_cmpxchg_local_
helpers which are not atomic.

Link: https://lkml.kernel.org/r/20230313162507.032200398@redhat.com
Link: https://lkml.kernel.org/r/20230313162634.283504192@redhat.com
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Aaron Tomlin <atomlin@atomlin.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Christian König <christian.koenig@amd.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lorenzo Stoakes <lstoakes@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

--- a/arch/arm64/include/asm/percpu.h~this_cpu_cmpxchg-arm64-switch-this_cpu_cmpxchg-to-locked-add-_local-function
+++ a/arch/arm64/include/asm/percpu.h
@@ -232,13 +232,23 @@ PERCPU_RET_OP(add, add, ldadd)
 	_pcp_protect_return(xchg_relaxed, pcp, val)
 
 #define this_cpu_cmpxchg_1(pcp, o, n)	\
-	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+	_pcp_protect_return(cmpxchg, pcp, o, n)
 #define this_cpu_cmpxchg_2(pcp, o, n)	\
-	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+	_pcp_protect_return(cmpxchg, pcp, o, n)
 #define this_cpu_cmpxchg_4(pcp, o, n)	\
-	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+	_pcp_protect_return(cmpxchg, pcp, o, n)
 #define this_cpu_cmpxchg_8(pcp, o, n)	\
+	_pcp_protect_return(cmpxchg, pcp, o, n)
+
+#define this_cpu_cmpxchg_local_1(pcp, o, n)	\
 	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+#define this_cpu_cmpxchg_local_2(pcp, o, n)	\
+	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+#define this_cpu_cmpxchg_local_4(pcp, o, n)	\
+	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+#define this_cpu_cmpxchg_local_8(pcp, o, n)	\
+	_pcp_protect_return(cmpxchg_relaxed, pcp, o, n)
+
 
 #ifdef __KVM_NVHE_HYPERVISOR__
 extern unsigned long __hyp_per_cpu_offset(unsigned int cpu);
_

Patches currently in -mm which might be from mtosatti@redhat.com are

this_cpu_cmpxchg-arm64-switch-this_cpu_cmpxchg-to-locked-add-_local-function.patch
this_cpu_cmpxchg-loongarch-switch-this_cpu_cmpxchg-to-locked-add-_local-function.patch
this_cpu_cmpxchg-s390-switch-this_cpu_cmpxchg-to-locked-add-_local-function.patch
this_cpu_cmpxchg-x86-switch-this_cpu_cmpxchg-to-locked-add-_local-function.patch
add-this_cpu_cmpxchg_local-and-asm-generic-definitions.patch
convert-this_cpu_cmpxchg-users-to-this_cpu_cmpxchg_local.patch
mm-vmstat-switch-counter-modification-to-cmpxchg.patch
vmstat-switch-per-cpu-vmstat-counters-to-32-bits.patch
mm-vmstat-use-xchg-in-cpu_vm_stats_fold.patch
mm-vmstat-switch-vmstat-shepherd-to-flush-per-cpu-counters-remotely.patch
mm-vmstat-refresh-stats-remotely-instead-of-via-work-item.patch
vmstat-add-pcp-remote-node-draining-via-cpu_vm_stats_fold.patch


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