This is a note to let you know that I've just added the patch titled
x86/irq: Do not substract irq_tlb_count from irq_call_count
to the 4.4-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git%3Ba=su...
The filename of the patch is: x86-irq-do-not-substract-irq_tlb_count-from-irq_call_count.patch and it can be found in the queue-4.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let stable@vger.kernel.org know about it.
From 82ba4faca1bffad429f15c90c980ffd010366c25 Mon Sep 17 00:00:00 2001
From: Aaron Lu aaron.lu@intel.com Date: Thu, 11 Aug 2016 15:44:30 +0800 Subject: x86/irq: Do not substract irq_tlb_count from irq_call_count
From: Aaron Lu aaron.lu@intel.com
commit 82ba4faca1bffad429f15c90c980ffd010366c25 upstream.
Since commit:
52aec3308db8 ("x86/tlb: replace INVALIDATE_TLB_VECTOR by CALL_FUNCTION_VECTOR")
the TLB remote shootdown is done through call function vector. That commit didn't take care of irq_tlb_count, which a later commit:
fd0f5869724f ("x86: Distinguish TLB shootdown interrupts from other functions call interrupts")
... tried to fix.
The fix assumes every increase of irq_tlb_count has a corresponding increase of irq_call_count. So the irq_call_count is always bigger than irq_tlb_count and we could substract irq_tlb_count from irq_call_count.
Unfortunately this is not true for the smp_call_function_single() case. The IPI is only sent if the target CPU's call_single_queue is empty when adding a csd into it in generic_exec_single. That means if two threads are both adding flush tlb csds to the same CPU's call_single_queue, only one IPI is sent. In other words, the irq_call_count is incremented by 1 but irq_tlb_count is incremented by 2. Over time, irq_tlb_count will be bigger than irq_call_count and the substract will produce a very large irq_call_count value due to overflow.
Considering that:
1) it's not worth to send more IPIs for the sake of accurate counting of irq_call_count in generic_exec_single();
2) it's not easy to tell if the call function interrupt is for TLB shootdown in __smp_call_function_single_interrupt().
Not to exclude TLB shootdown from call function count seems to be the simplest fix and this patch just does that.
This bug was found by LKP's cyclic performance regression tracking recently with the vm-scalability test suite. I have bisected to commit:
3dec0ba0be6a ("mm/rmap: share the i_mmap_rwsem")
This commit didn't do anything wrong but revealed the irq_call_count problem. IIUC, the commit makes rwc->remap_one in rmap_walk_file concurrent with multiple threads. When remap_one is try_to_unmap_one(), then multiple threads could queue flush TLB to the same CPU but only one IPI will be sent.
Since the commit was added in Linux v3.19, the counting problem only shows up from v3.19 onwards.
Signed-off-by: Aaron Lu aaron.lu@intel.com Cc: Alex Shi alex.shi@linaro.org Cc: Andy Lutomirski luto@kernel.org Cc: Borislav Petkov bp@alien8.de Cc: Brian Gerst brgerst@gmail.com Cc: Davidlohr Bueso dave@stgolabs.net Cc: Denys Vlasenko dvlasenk@redhat.com Cc: H. Peter Anvin hpa@zytor.com Cc: Huang Ying ying.huang@intel.com Cc: Josh Poimboeuf jpoimboe@redhat.com Cc: Linus Torvalds torvalds@linux-foundation.org Cc: Peter Zijlstra peterz@infradead.org Cc: Thomas Gleixner tglx@linutronix.de Cc: Tomoki Sekiyama tomoki.sekiyama.qu@hitachi.com Link: http://lkml.kernel.org/r/20160811074430.GA18163@aaronlu.sh.intel.com Signed-off-by: Ingo Molnar mingo@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/include/asm/hardirq.h | 4 ---- arch/x86/kernel/irq.c | 3 +-- 2 files changed, 1 insertion(+), 6 deletions(-)
--- a/arch/x86/include/asm/hardirq.h +++ b/arch/x86/include/asm/hardirq.h @@ -22,10 +22,6 @@ typedef struct { #ifdef CONFIG_SMP unsigned int irq_resched_count; unsigned int irq_call_count; - /* - * irq_tlb_count is double-counted in irq_call_count, so it must be - * subtracted from irq_call_count when displaying irq_call_count - */ unsigned int irq_tlb_count; #endif #ifdef CONFIG_X86_THERMAL_VECTOR --- a/arch/x86/kernel/irq.c +++ b/arch/x86/kernel/irq.c @@ -102,8 +102,7 @@ int arch_show_interrupts(struct seq_file seq_puts(p, " Rescheduling interrupts\n"); seq_printf(p, "%*s: ", prec, "CAL"); for_each_online_cpu(j) - seq_printf(p, "%10u ", irq_stats(j)->irq_call_count - - irq_stats(j)->irq_tlb_count); + seq_printf(p, "%10u ", irq_stats(j)->irq_call_count); seq_puts(p, " Function call interrupts\n"); seq_printf(p, "%*s: ", prec, "TLB"); for_each_online_cpu(j)
Patches currently in stable-queue which might be from aaron.lu@intel.com are
queue-4.4/x86-irq-do-not-substract-irq_tlb_count-from-irq_call_count.patch