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authorMel Gorman <mgorman@suse.de>2012-03-21 16:34:11 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2012-03-21 17:54:59 -0700
commitcc9a6c8776615f9c194ccf0b63a0aa5628235545 (patch)
tree0cbbf118e86541f8eb2fc7b717a0e08eaced986d /include/linux/cpuset.h
parente845e199362cc5712ba0e7eedc14eed70e144258 (diff)
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cpuset: mm: reduce large amounts of memory barrier related damage v3
Commit c0ff7453bb5c ("cpuset,mm: fix no node to alloc memory when changing cpuset's mems") wins a super prize for the largest number of memory barriers entered into fast paths for one commit. [get|put]_mems_allowed is incredibly heavy with pairs of full memory barriers inserted into a number of hot paths. This was detected while investigating at large page allocator slowdown introduced some time after 2.6.32. The largest portion of this overhead was shown by oprofile to be at an mfence introduced by this commit into the page allocator hot path. For extra style points, the commit introduced the use of yield() in an implementation of what looks like a spinning mutex. This patch replaces the full memory barriers on both read and write sides with a sequence counter with just read barriers on the fast path side. This is much cheaper on some architectures, including x86. The main bulk of the patch is the retry logic if the nodemask changes in a manner that can cause a false failure. While updating the nodemask, a check is made to see if a false failure is a risk. If it is, the sequence number gets bumped and parallel allocators will briefly stall while the nodemask update takes place. In a page fault test microbenchmark, oprofile samples from __alloc_pages_nodemask went from 4.53% of all samples to 1.15%. The actual results were 3.3.0-rc3 3.3.0-rc3 rc3-vanilla nobarrier-v2r1 Clients 1 UserTime 0.07 ( 0.00%) 0.08 (-14.19%) Clients 2 UserTime 0.07 ( 0.00%) 0.07 ( 2.72%) Clients 4 UserTime 0.08 ( 0.00%) 0.07 ( 3.29%) Clients 1 SysTime 0.70 ( 0.00%) 0.65 ( 6.65%) Clients 2 SysTime 0.85 ( 0.00%) 0.82 ( 3.65%) Clients 4 SysTime 1.41 ( 0.00%) 1.41 ( 0.32%) Clients 1 WallTime 0.77 ( 0.00%) 0.74 ( 4.19%) Clients 2 WallTime 0.47 ( 0.00%) 0.45 ( 3.73%) Clients 4 WallTime 0.38 ( 0.00%) 0.37 ( 1.58%) Clients 1 Flt/sec/cpu 497620.28 ( 0.00%) 520294.53 ( 4.56%) Clients 2 Flt/sec/cpu 414639.05 ( 0.00%) 429882.01 ( 3.68%) Clients 4 Flt/sec/cpu 257959.16 ( 0.00%) 258761.48 ( 0.31%) Clients 1 Flt/sec 495161.39 ( 0.00%) 517292.87 ( 4.47%) Clients 2 Flt/sec 820325.95 ( 0.00%) 850289.77 ( 3.65%) Clients 4 Flt/sec 1020068.93 ( 0.00%) 1022674.06 ( 0.26%) MMTests Statistics: duration Sys Time Running Test (seconds) 135.68 132.17 User+Sys Time Running Test (seconds) 164.2 160.13 Total Elapsed Time (seconds) 123.46 120.87 The overall improvement is small but the System CPU time is much improved and roughly in correlation to what oprofile reported (these performance figures are without profiling so skew is expected). The actual number of page faults is noticeably improved. For benchmarks like kernel builds, the overall benefit is marginal but the system CPU time is slightly reduced. To test the actual bug the commit fixed I opened two terminals. The first ran within a cpuset and continually ran a small program that faulted 100M of anonymous data. In a second window, the nodemask of the cpuset was continually randomised in a loop. Without the commit, the program would fail every so often (usually within 10 seconds) and obviously with the commit everything worked fine. With this patch applied, it also worked fine so the fix should be functionally equivalent. Signed-off-by: Mel Gorman <mgorman@suse.de> Cc: Miao Xie <miaox@cn.fujitsu.com> Cc: David Rientjes <rientjes@google.com> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Christoph Lameter <cl@linux.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/cpuset.h')
-rw-r--r--include/linux/cpuset.h47
1 files changed, 20 insertions, 27 deletions
diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index e9eaec522655..7a7e5fd2a277 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -89,42 +89,33 @@ extern void rebuild_sched_domains(void);
extern void cpuset_print_task_mems_allowed(struct task_struct *p);
/*
- * reading current mems_allowed and mempolicy in the fastpath must protected
- * by get_mems_allowed()
+ * get_mems_allowed is required when making decisions involving mems_allowed
+ * such as during page allocation. mems_allowed can be updated in parallel
+ * and depending on the new value an operation can fail potentially causing
+ * process failure. A retry loop with get_mems_allowed and put_mems_allowed
+ * prevents these artificial failures.
*/
-static inline void get_mems_allowed(void)
+static inline unsigned int get_mems_allowed(void)
{
- current->mems_allowed_change_disable++;
-
- /*
- * ensure that reading mems_allowed and mempolicy happens after the
- * update of ->mems_allowed_change_disable.
- *
- * the write-side task finds ->mems_allowed_change_disable is not 0,
- * and knows the read-side task is reading mems_allowed or mempolicy,
- * so it will clear old bits lazily.
- */
- smp_mb();
+ return read_seqcount_begin(&current->mems_allowed_seq);
}
-static inline void put_mems_allowed(void)
+/*
+ * If this returns false, the operation that took place after get_mems_allowed
+ * may have failed. It is up to the caller to retry the operation if
+ * appropriate.
+ */
+static inline bool put_mems_allowed(unsigned int seq)
{
- /*
- * ensure that reading mems_allowed and mempolicy before reducing
- * mems_allowed_change_disable.
- *
- * the write-side task will know that the read-side task is still
- * reading mems_allowed or mempolicy, don't clears old bits in the
- * nodemask.
- */
- smp_mb();
- --ACCESS_ONCE(current->mems_allowed_change_disable);
+ return !read_seqcount_retry(&current->mems_allowed_seq, seq);
}
static inline void set_mems_allowed(nodemask_t nodemask)
{
task_lock(current);
+ write_seqcount_begin(&current->mems_allowed_seq);
current->mems_allowed = nodemask;
+ write_seqcount_end(&current->mems_allowed_seq);
task_unlock(current);
}
@@ -234,12 +225,14 @@ static inline void set_mems_allowed(nodemask_t nodemask)
{
}
-static inline void get_mems_allowed(void)
+static inline unsigned int get_mems_allowed(void)
{
+ return 0;
}
-static inline void put_mems_allowed(void)
+static inline bool put_mems_allowed(unsigned int seq)
{
+ return true;
}
#endif /* !CONFIG_CPUSETS */