x86, mm: trace when an IPI is about to be sent

Mel Gorman · torvalds · commit 5b74283ab251 · 2015-09-04T16:54:41.000-07:00
When unmapping pages it is necessary to flush the TLB.  If that page was
accessed by another CPU then an IPI is used to flush the remote CPU.  That
is a lot of IPIs if kswapd is scanning and unmapping &gt;100K pages per
second.

There already is a window between when a page is unmapped and when it is
TLB flushed.  This series increases the window so multiple pages can be
flushed using a single IPI.  This should be safe or the kernel is hosed
already.

Patch 1 simply made the rest of the series easier to write as ftrace
        could identify all the senders of TLB flush IPIS.

Patch 2 tracks what CPUs potentially map a PFN and then sends an IPI
        to flush the entire TLB.

Patch 3 tracks when there potentially are writable TLB entries that
        need to be batched differently

Patch 4 increases SWAP_CLUSTER_MAX to further batch flushes

The performance impact is documented in the changelogs but in the optimistic
case on a 4-socket machine the full series reduces interrupts from 900K
interrupts/second to 60K interrupts/second.

This patch (of 4):

It is easy to trace when an IPI is received to flush a TLB but harder to
detect what event sent it.  This patch makes it easy to identify the
source of IPIs being transmitted for TLB flushes on x86.

Signed-off-by: Mel Gorman &lt;mgorman@suse.de&gt;
Reviewed-by: Rik van Riel &lt;riel@redhat.com&gt;
Reviewed-by: Dave Hansen &lt;dave.hansen@intel.com&gt;
Acked-by: Ingo Molnar &lt;mingo@kernel.org&gt;
Cc: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
Signed-off-by: Andrew Morton &lt;akpm@linux-foundation.org&gt;
Signed-off-by: Linus Torvalds &lt;torvalds@linux-foundation.org&gt;
diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
@@ -140,6 +140,7 @@ void native_flush_tlb_others(const struct cpumask *cpumask,
 	info.flush_end = end;
 
 	count_vm_tlb_event(NR_TLB_REMOTE_FLUSH);
+	trace_tlb_flush(TLB_REMOTE_SEND_IPI, end - start);
 	if (is_uv_system()) {
 		unsigned int cpu;
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
@@ -554,6 +554,7 @@ enum tlb_flush_reason {
 	TLB_REMOTE_SHOOTDOWN,
 	TLB_LOCAL_SHOOTDOWN,
 	TLB_LOCAL_MM_SHOOTDOWN,
+	TLB_REMOTE_SEND_IPI,
 	NR_TLB_FLUSH_REASONS,
 };
 
diff --git a/include/trace/events/tlb.h b/include/trace/events/tlb.h
@@ -11,7 +11,8 @@
 	EM(  TLB_FLUSH_ON_TASK_SWITCH,	"flush on task switch" )	\
 	EM(  TLB_REMOTE_SHOOTDOWN,	"remote shootdown" )		\
 	EM(  TLB_LOCAL_SHOOTDOWN,	"local shootdown" )		\
-	EMe( TLB_LOCAL_MM_SHOOTDOWN,	"local mm shootdown" )
+	EM(  TLB_LOCAL_MM_SHOOTDOWN,	"local mm shootdown" )		\
+	EMe( TLB_REMOTE_SEND_IPI,	"remote ipi send" )
 
 /*
  * First define the enums in TLB_FLUSH_REASON to be exported to userspace
