linux-kernel - Re: [PATCH v6 0/4] Add support for length of each segment in the segcblist

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Message-ID: <20200924234212.GZ29330@paulmck-ThinkPad-P72>
Date:   Thu, 24 Sep 2020 16:42:12 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        neeraj.iitr10@...il.com, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>
Subject: Re: [PATCH v6 0/4] Add support for length of each segment in the
 segcblist

On Wed, Sep 23, 2020 at 11:22:07AM -0400, Joel Fernandes (Google) wrote:
> 
> This is required for several usecases identified. One of them being tracing how
> the segmented callback list changes. Tracing this has identified issues in RCU
> code in the past.
> 
> >From Paul:
> Another use case is of course more accurately determining whether a given CPU's
> large pile of callbacks can be best served by making grace periods go faster,
> invoking callbacks more vigorously, or both.  It should also be possible to
> simplify some of the callback handling a bit, given that some of the unnatural
> acts are due to there having been no per-batch counts.
> 
> Revision history:
> v6: Fixed TREE04, and restored older logic to ensure rcu_barrier works.
> 
> v5: Various changes, bug fixes. Discovery of rcu_barrier issue.
> 
> v4: Restructured rcu_do_batch() and segcblist merging to avoid issues.
>     Fixed minor nit from Davidlohr.
> v1->v3: minor nits.
> (https://lore.kernel.org/lkml/20200719034210.2382053-1-joel@joelfernandes.org/)

Looking much improved, thank you!

I have placed these on branch rcu/test in -rcu for testing and inspection.
I had to apply them at b94e6291a208 ("torture: Force weak-hashed pointers
on console log") and cherry-pick them onto the "dev" branch, but it looks
like things worked nicely.

							Thanx, Paul

> Joel Fernandes (Google) (4):
> rcu/tree: Make rcu_do_batch count how many callbacks were executed
> rcu/segcblist: Add counters to segcblist datastructure
> rcu/trace: Add tracing for how segcb list changes
> rcu/segcblist: Remove useless rcupdate.h include
> 
> include/linux/rcu_segcblist.h |   2 +
> include/trace/events/rcu.h    |  25 ++++++
> kernel/rcu/rcu_segcblist.c    | 161 +++++++++++++++++++++++++---------
> kernel/rcu/rcu_segcblist.h    |   8 +-
> kernel/rcu/tree.c             |  18 ++--
> 5 files changed, 165 insertions(+), 49 deletions(-)
> 
> --
> 2.28.0.681.g6f77f65b4e-goog
>