Re: [PATCH 1/7] rcu/segcblist: Prevent useless GP start if no CBs to accelerate

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Jun 18, 2020 at 04:29:49PM -0400, Joel Fernandes (Google) wrote:

First, this looks like a very nice optimization, thank you!

> rcu_segcblist_accelerate() returns true if a GP is to be
> started/requested and false if not. During tracing, I found that it is
> asking that GPs be requested

s/GPs/unnecessary GPs/?  Plus "." at end of the sentence.

> The exact flow seems to be something like:
> 1. Callbacks are queued on CPU A - into the NEXT list.
> 2. softirq runs on CPU A, accelerate all CBs from NEXT->WAIT and request a GP X.
> 3. GP thread wakes up on another CPU, starts the GP X and requests QS from CPU A.
> 4. CPU A's softirq runs again presumably because of #3.

Yes, that is one reason RCU softirq might run again.

> 5. CPU A's softirq now does acceleration again, this time no CBs are
>    accelerated since last attempt, but it still requests GP X+1 which
>    could be useless.

I can't think of a case where this request helps.  How about: "but
it still unnecessarily requests GP X+1"?

> The fix is, prevent the useless GP start if we detect no CBs are there
> to accelerate.
> 
> With this, we have the following improvement in short runs of
> rcutorture (5 seconds each):
> +----+-------------------+-------------------+
> | #  | Number of GPs     | Number of Wakeups |
> +====+=========+=========+=========+=========+
> | 1  | With    | Without | With    | Without |
> +----+---------+---------+---------+---------+
> | 2  |      75 |      89 |     113 |     119 |
> +----+---------+---------+---------+---------+
> | 3  |      62 |      91 |     105 |     123 |
> +----+---------+---------+---------+---------+
> | 4  |      60 |      79 |      98 |     110 |
> +----+---------+---------+---------+---------+
> | 5  |      63 |      79 |      99 |     112 |
> +----+---------+---------+---------+---------+
> | 6  |      57 |      89 |      96 |     123 |
> +----+---------+---------+---------+---------+
> | 7  |      64 |      85 |      97 |     118 |
> +----+---------+---------+---------+---------+
> | 8  |      58 |      83 |      98 |     113 |
> +----+---------+---------+---------+---------+
> | 9  |      57 |      77 |      89 |     104 |
> +----+---------+---------+---------+---------+
> | 10 |      66 |      82 |      98 |     119 |
> +----+---------+---------+---------+---------+
> | 11 |      52 |      82 |      83 |     117 |
> +----+---------+---------+---------+---------+

So the reductions in wakeups ranges from 5% to 40%, with almost a 20%
overall reduction in wakeups across all the runs.  That should be of
some use to someone.  ;-)

I do run rcutorture quite a bit, but is there a more real-world
benchmark that could be tried?

> Cc: urezki@gmail.com
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
>  kernel/rcu/rcu_segcblist.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
> index 9a0f66133b4b3..4782cf17bf4f9 100644
> --- a/kernel/rcu/rcu_segcblist.c
> +++ b/kernel/rcu/rcu_segcblist.c
> @@ -475,8 +475,15 @@ bool rcu_segcblist_accelerate(struct rcu_segcblist *rsclp, unsigned long seq)
>  	 * Also advance to the oldest segment of callbacks whose
>  	 * ->gp_seq[] completion is at or after that passed in via "seq",
>  	 * skipping any empty segments.
> +	 *
> +	 * Note that "i" is the youngest segment of the list after which
> +	 * any older segments than "i" would not be mutated or assigned
> +	 * GPs. For example, if i == WAIT_TAIL, then neither WAIT_TAIL,
> +	 * nor DONE_TAIL will be touched. Only CBs in NEXT_TAIL will be
> +	 * merged with NEXT_READY_TAIL and the GP numbers of both of
> +	 * them would be updated.

In this case, only the GP number of NEXT_READY_TAIL would be updated,
correct?  Or am I missing something subtle in the loop just past the
end of this patch?

							Thanx, Paul

>  	 */
> -	if (++i >= RCU_NEXT_TAIL)
> +	if (rcu_segcblist_restempty(rsclp, i) || ++i >= RCU_NEXT_TAIL)
>  		return false;
>  
>  	/*
> -- 
> 2.27.0.111.gc72c7da667-goog
>