Re: [PATCH] rcu: Avoid unneeded function call in rcu_read_unlock()
From: Waiman Long
Date: Fri Aug 27 2021 - 16:27:10 EST
On 8/27/21 2:34 PM, Paul E. McKenney wrote:
On Thu, Aug 26, 2021 at 10:21:22PM -0400, Waiman Long wrote:
Since commit aa40c138cc8f3 ("rcu: Report QS for outermost
PREEMPT=n rcu_read_unlock() for strict GPs"). A real function call
rcu_read_unlock_strict() is added to the inlined rcu_read_unlock().
The rcu_read_unlock_strict() call is only needed if the performance
sagging CONFIG_RCU_STRICT_GRACE_PERIOD option is set. This config
option isn't set for most production kernels while the function call
overhead remains.
To provide a slight performance improvement, the
CONFIG_RCU_STRICT_GRACE_PERIOD config check is moved from
rcu_read_unlock_strict() to __rcu_read_unlock() so that the function
call can be compiled out in most cases.
Besides, the GPL exported rcu_read_unlock_strict() also impact the
the compilation of non-GPL kernel modules as rcu_read_unlock() is a
frequently used kernel API.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Nice, and good eyes!!!
I have queued this for v5.16, that is, not the upcoming merge window
but the one after that.
I did my usual wordsmithing, so please check the following in case I
messed something up. I intentionally omitted the EXPORT_SYMBOL_GPL()
discussion because:
1. Kernels built with CONFIG_PREEMPT=y have the same issue
with the __rcu_read_lock() and __rcu_read_unlock() functions.
2. Many other RCU functions are EXPORT_SYMBOL_GPL() and have
been for almost two decades.
But if someone does use RCU readers within CONFIG_PREEMPT=n kernels from
a binary module, I will happily refer them to you for any RCU issues
that they encounter. ;-)
I am also CCing the BPF guys in case my interpretation of the code in
the BPF verifier is incorrect.
Thanx, Paul
It looks good to me. Thanks for the rewording. I did regret mentioning
about about the GPL export symbol in the commit log and it is good that
you had taken it out.
Cheers,
Longman