+ kthread-simplify-kthread_use_mm-refcounting.patch added to mm-unstable branch

+ kthread-simplify-kthread_use_mm-refcounting.patch added to mm-unstable branch

[Andrew Morton]

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

Before you just go and hit "reply", please:
   a) Consider who else should be cc'ed
   b) Prefer to cc a suitable mailing list as well
   c) Ideally: find the original patch on the mailing list and do a
      reply-to-all to that, adding suitable additional cc's

*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***

The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days

------------------------------------------------------
From: Nicholas Piggin <npiggin@gmail.com>
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@gmail.com/


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@gmail.com
Link: https://lkml.kernel.org/r/20230203071837.1136453-2-npiggin@gmail.com
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Will Deacon <will@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---


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@gmail.com 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