kthread: simplify kthread_use_mm refcounting
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>
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@ -1415,14 +1415,13 @@ void kthread_use_mm(struct mm_struct *mm)
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WARN_ON_ONCE(!(tsk->flags & PF_KTHREAD));
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WARN_ON_ONCE(tsk->mm);
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mmgrab(mm);
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task_lock(tsk);
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/* Hold off tlb flush IPIs while switching mm's */
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local_irq_disable();
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active_mm = tsk->active_mm;
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if (active_mm != mm) {
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mmgrab(mm);
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tsk->active_mm = mm;
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}
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tsk->active_mm = mm;
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tsk->mm = mm;
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membarrier_update_current_mm(mm);
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switch_mm_irqs_off(active_mm, mm, tsk);
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@ -1439,12 +1438,9 @@ void kthread_use_mm(struct mm_struct *mm)
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* memory barrier after storing to tsk->mm, before accessing
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* user-space memory. A full memory barrier for membarrier
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* {PRIVATE,GLOBAL}_EXPEDITED is implicitly provided by
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* mmdrop(), or explicitly with smp_mb().
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* mmdrop().
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*/
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if (active_mm != mm)
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mmdrop(active_mm);
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else
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smp_mb();
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mmdrop(active_mm);
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}
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EXPORT_SYMBOL_GPL(kthread_use_mm);
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