The patch titled
Subject: kthread: simplify kthread_use_mm refcounting
has been added to the -mm mm-unstable branch. Its filename is
kthread-simplify-kthread_use_mm-refcounting.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/kthread-simplify-kthread_use_mm-refcounting.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: Nicholas Piggin <npiggin@xxxxxxxxx>
Subject: kthread: simplify kthread_use_mm refcounting
Date: Fri, 3 Feb 2023 17:18:33 +1000
Patch series "shoot lazy tlbs (lazy tlb refcount scalability
improvement)", v7.
This series improves scalability of context switching between user and
kernel threads on large systems with a threaded process spread across a
lot of CPUs.
Discussion of v6 here:
https://lore.kernel.org/linux-mm/20230118080011.2258375-1-npiggin@xxxxxxxxx/
This patch (of 5):
Remove the special case avoiding refcounting when the mm to be used is the
same as the kernel thread's active (lazy tlb) mm. kthread_use_mm() should
not be such a performance critical path that this matters much. This
simplifies a later change to lazy tlb mm refcounting.
Link: https://lkml.kernel.org/r/20230203071837.1136453-1-npiggin@xxxxxxxxx
Link: https://lkml.kernel.org/r/20230203071837.1136453-2-npiggin@xxxxxxxxx
Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx>
Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
diff --git a/kernel/kthread.c~kthread-simplify-kthread_use_mm-refcounting b/kernel/kthread.c
--- a/kernel/kthread.c~kthread-simplify-kthread_use_mm-refcounting
+++ b/kernel/kthread.c
@@ -1415,14 +1415,13 @@ void kthread_use_mm(struct mm_struct *mm)
WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
WARN_ON_ONCE(tsk->mm);
+ mmgrab(mm);
+
task_lock(tsk);
/* Hold off tlb flush IPIs while switching mm's */
local_irq_disable();
active_mm = tsk->active_mm;
- if (active_mm != mm) {
- mmgrab(mm);
- tsk->active_mm = mm;
- }
+ tsk->active_mm = mm;
tsk->mm = mm;
membarrier_update_current_mm(mm);
switch_mm_irqs_off(active_mm, mm, tsk);
@@ -1439,12 +1438,9 @@ void kthread_use_mm(struct mm_struct *mm)
* memory barrier after storing to tsk->mm, before accessing
* user-space memory. A full memory barrier for membarrier
* {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by
- * mmdrop(), or explicitly with smp_mb().
+ * mmdrop().
*/
- if (active_mm != mm)
- mmdrop(active_mm);
- else
- smp_mb();
+ mmdrop(active_mm);
}
EXPORT_SYMBOL_GPL(kthread_use_mm);
_
Patches currently in -mm which might be from npiggin@xxxxxxxxx are
kthread-simplify-kthread_use_mm-refcounting.patch
lazy-tlb-introduce-lazy-tlb-mm-refcount-helper-functions.patch
lazy-tlb-allow-lazy-tlb-mm-refcounting-to-be-configurable.patch
lazy-tlb-shoot-lazies-non-refcounting-lazy-tlb-mm-reference-handling-scheme.patch
powerpc-64s-enable-mmu_lazy_tlb_shootdown.patch
