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  1. 17 Nov, 2019 1 commit
    • Sebastian Andrzej Siewior's avatar
      percpu-refcount: Use normal instead of RCU-sched" · 9e8d42a0
      Sebastian Andrzej Siewior authored 
      This is a revert of commit
   a4244454 ("percpu-refcount: use RCU-sched insted of normal RCU")

which claims the only reason for using RCU-sched is
   "rcu_read_[un]lock() … are slightly more expensive than preempt_disable/enable()"

and
    "As the RCU critical sections are extremely short, using sched-RCU
    shouldn't have any latency implications."

The problem with using RCU-sched here is that it disables preemption and
the release callback (called from percpu_ref_put_many()) must not
acquire any sleeping locks like spinlock_t. This breaks PREEMPT_RT
because some of the users acquire spinlock_t locks in their callbacks.

Using rcu_read_lock() on PREEMPTION=n kernels is not any different
compared to rcu_read_lock_sched(). On PREEMPTION=y kernels there are
already performance issues due to additional preemption points.
Looking at the code, the rcu_read_lock() is just an increment and unlock
is almost just a decrement unless there is something special to do. Both
are functions while disabling preemption is inlined.
Doing a small benchmark, the minimal amount of time required was mostly
the same. The average time required was higher due to the higher MAX
value (which could be preemption). With DEBUG_PREEMPT=y it is
rcu_read_lock_sched() that takes a little longer due to the additional
debug code.

Convert back to normal RCU.
Signed-off-by: default avatarSebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: default avatarDennis Zhou <dennis@kernel.org>
      9e8d42a0
  3. 17 Oct, 2019 1 commit
    • Ben Dooks's avatar
      percpu: add __percpu to SHIFT_PERCPU_PTR · 825dbc6f
      Ben Dooks authored 
      The SHIFT_PERCPU_PTR() returns a pointer used by a number
of functions that expect the pointer to be __percpu annotated
(sparse address space 3). Adding __percpu to this makes the
following sparse warnings go away.

Note, this then creates the problem the __percup is marked
as noderef, which may need removing for some of the internal
functions, or to remove other warnings.

mm/vmstat.c:385:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:385:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:385:13:    got signed char *
mm/vmstat.c:385:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:385:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:385:13:    got signed char *
mm/vmstat.c:385:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:385:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:385:13:    got signed char *
mm/vmstat.c:385:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:385:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:385:13:    got signed char *
mm/vmstat.c:401:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:401:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:401:13:    got signed char *
mm/vmstat.c:401:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:401:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:401:13:    got signed char *
mm/vmstat.c:401:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:401:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:401:13:    got signed char *
mm/vmstat.c:401:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:401:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:401:13:    got signed char *
mm/vmstat.c:429:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:429:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:429:13:    got signed char *
mm/vmstat.c:429:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:429:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:429:13:    got signed char *
mm/vmstat.c:429:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:429:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:429:13:    got signed char *
mm/vmstat.c:429:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:429:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:429:13:    got signed char *
mm/vmstat.c:445:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:445:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:445:13:    got signed char *
mm/vmstat.c:445:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:445:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:445:13:    got signed char *
mm/vmstat.c:445:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:445:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:445:13:    got signed char *
mm/vmstat.c:445:13: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:445:13:    expected signed char [noderef] [usertype] <asn:3> *__p
mm/vmstat.c:445:13:    got signed char *
mm/vmstat.c:763:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:763:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:763:29:    got signed char *
mm/vmstat.c:763:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:763:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:763:29:    got signed char *
mm/vmstat.c:763:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:763:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:763:29:    got signed char *
mm/vmstat.c:763:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:763:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:763:29:    got signed char *
mm/vmstat.c:825:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:825:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:825:29:    got signed char *
mm/vmstat.c:825:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:825:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:825:29:    got signed char *
mm/vmstat.c:825:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:825:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:825:29:    got signed char *
mm/vmstat.c:825:29: warning: incorrect type in initializer (different address spaces)
mm/vmstat.c:825:29:    expected signed char [noderef] <asn:3> *__p
mm/vmstat.c:825:29:    got signed char *
Signed-off-by: default avatarBen Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: default avatarDennis Zhou <dennis@kernel.org>
      825dbc6f
  5. 13 Oct, 2019 16 commits
  7. 12 Oct, 2019 22 commits
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