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McKenney" , rcu@vger.kernel.org, Steven Rostedt , "Uladzislau Rezki (Sony)" , fweisbec@gmail.com, neeraj.iitr10@gmail.com Subject: Re: [PATCH v7 2/6] rcu/segcblist: Add counters to segcblist datastructure Message-ID: <20201017013123.GC4015033@google.com> References: <20201015002301.101830-1-joel@joelfernandes.org> <20201015002301.101830-3-joel@joelfernandes.org> <20201015122158.GA127222@lothringen> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201015122158.GA127222@lothringen> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 15, 2020 at 02:21:58PM +0200, Frederic Weisbecker wrote: > On Wed, Oct 14, 2020 at 08:22:57PM -0400, Joel Fernandes (Google) wrote: > > Add counting of segment lengths of segmented callback list. > > > > This will be useful for a number of things such as knowing how big the > > ready-to-execute segment have gotten. The immediate benefit is ability > > to trace how the callbacks in the segmented callback list change. > > > > Also this patch remove hacks related to using donecbs's ->len field as a > > temporary variable to save the segmented callback list's length. This cannot be > > done anymore and is not needed. > > > > Signed-off-by: Joel Fernandes (Google) > > --- > > include/linux/rcu_segcblist.h | 2 + > > kernel/rcu/rcu_segcblist.c | 133 +++++++++++++++++++++++----------- > > kernel/rcu/rcu_segcblist.h | 2 - > > 3 files changed, 92 insertions(+), 45 deletions(-) > > > > diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h > > index b36afe7b22c9..d462ae5e340a 100644 > > --- a/include/linux/rcu_segcblist.h > > +++ b/include/linux/rcu_segcblist.h > > @@ -69,8 +69,10 @@ struct rcu_segcblist { > > unsigned long gp_seq[RCU_CBLIST_NSEGS]; > > #ifdef CONFIG_RCU_NOCB_CPU > > atomic_long_t len; > > + atomic_long_t seglen[RCU_CBLIST_NSEGS]; > > Also does it really need to be atomic? Yeah I think not. In fact, I am not even sure if ->len in the existing code needs to be atomic. I am considering vetting all code paths. Paul told me it used to be the case that ->len could be lockless written (possibly without IRQs disabled) I think but that maybe nowadays it is not the case. Let us double check to be sure. > > @@ -245,7 +280,7 @@ void rcu_segcblist_enqueue(struct rcu_segcblist *rsclp, > > struct rcu_head *rhp) > > { > > rcu_segcblist_inc_len(rsclp); > > - smp_mb(); /* Ensure counts are updated before callback is enqueued. */ > > That's a significant change that shouldn't be hidden and unexplained in an unrelated > patch or it may be easily missed. I'd suggest to move this line together in > "rcu/tree: Remove redundant smp_mb() in rcu_do_batch" (with the title updated perhaps) > and maybe put it in the beginning of the series? Ok I can do that. > > + rcu_segcblist_inc_seglen(rsclp, RCU_NEXT_TAIL); > > rhp->next = NULL; > > WRITE_ONCE(*rsclp->tails[RCU_NEXT_TAIL], rhp); > > WRITE_ONCE(rsclp->tails[RCU_NEXT_TAIL], &rhp->next); > [...] > > @@ -330,11 +353,16 @@ void rcu_segcblist_extract_pend_cbs(struct rcu_segcblist *rsclp, > > > > if (!rcu_segcblist_pend_cbs(rsclp)) > > return; /* Nothing to do. */ > > + rclp->len = rcu_segcblist_get_seglen(rsclp, RCU_WAIT_TAIL) + > > + rcu_segcblist_get_seglen(rsclp, RCU_NEXT_READY_TAIL) + > > + rcu_segcblist_get_seglen(rsclp, RCU_NEXT_TAIL); > > *rclp->tail = *rsclp->tails[RCU_DONE_TAIL]; > > rclp->tail = rsclp->tails[RCU_NEXT_TAIL]; > > WRITE_ONCE(*rsclp->tails[RCU_DONE_TAIL], NULL); > > - for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++) > > + for (i = RCU_DONE_TAIL + 1; i < RCU_CBLIST_NSEGS; i++) { > > WRITE_ONCE(rsclp->tails[i], rsclp->tails[RCU_DONE_TAIL]); > > + rcu_segcblist_set_seglen(rsclp, i, 0); > > + } > > So, that's probably just a matter of personal preference, so feel free to > ignore but I'd rather do: > > rclp->len += rcu_segcblist_get_seglen(rsclp, i); > rcu_segcblist_set_seglen(rsclp, i, 0); > > instead of the big addition above. That way, if a new index ever gets added/renamed > to the segcblist, we'll take it into account. Also that spares a few lines. Yeah your way is better, I will do it this way. thanks, - Joel