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

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 >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 <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
index 90b924acd9822ffdd9409b9fd97325ea7954b97a..8ddb5d0d66fb6f6353e735760489a47a0b98dea7 100644 (file)
--- 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
index 26a30c3566f0c751b66174fe8813a005f00a817d..c8d0a73d64c455f95ba9ed3a0d855bcbc4d839ce 100644 (file)
--- 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
index 4250f364a6caad06a436636bf2a33a05aa442dbf..bc8815f45f3b384e411738664ff0d3fa1b99bc0b 100644 (file)
--- 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