Re: [PATCH 04/10] mm, page_alloc: Remove unnecessary taking of a seqlock when cpusets are disabled

[Vlastimil Babka]

On 07/20/2015 10:00 AM, Mel Gorman wrote:
From: Mel Gorman <mgor...@suse.de>
There is a seqcounter that protects spurious allocation fails when a task
is changing the allowed nodes in a cpuset. There is no need to check the
seqcounter until a cpuset exists.

If cpusets become enabled betwen _begin and _retry, then it will retry 
due to comparing with 0, but not crash, so it's safe.

Signed-off-by: Mel Gorman <mgor...@sujse.de>

Acked-by: Vlastimil Babka <vba...@suse.cz>

---
  include/linux/cpuset.h | 6 ++++++
  1 file changed, 6 insertions(+)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 1b357997cac5..6eb27cb480b7 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -104,6 +104,9 @@ extern void cpuset_print_task_mems_allowed(struct 
task_struct *p);
   */
  static inline unsigned int read_mems_allowed_begin(void)
  {
+       if (!cpusets_enabled())
+               return 0;
+
        return read_seqcount_begin(&current->mems_allowed_seq);
  }

@@ -115,6 +118,9 @@ static inline unsigned int read_mems_allowed_begin(void)
   */
  static inline bool read_mems_allowed_retry(unsigned int seq)
  {
+       if (!cpusets_enabled())
+               return false;
+
        return read_seqcount_retry(&current->mems_allowed_seq, seq);
  }



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