linux-kernel - Re: [PATCH v2 tip/core/rcu 6/6] srcu: Document polling interfaces for Tree SRCU grace periods

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Date:   Fri, 27 Nov 2020 13:57:10 +0530
From:   Neeraj Upadhyay <neeraju@...eaurora.org>
To:     paulmck@...nel.org, rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org, kent.overstreet@...il.com
Subject: Re: [PATCH v2 tip/core/rcu 6/6] srcu: Document polling interfaces for
 Tree SRCU grace periods



On 11/21/2020 6:29 AM, paulmck@...nel.org wrote:
> From: "Paul E. McKenney" <paulmck@...nel.org>
> 
> This commit adds requirements documentation for the
> get_state_synchronize_srcu(), start_poll_synchronize_srcu(), and
> poll_state_synchronize_srcu() functions.
> 
> Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
> Reported-by: Kent Overstreet <kent.overstreet@...il.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---

Reviewed-by: Neeraj Upadhyay <neeraju@...eaurora.org>


Thanks
Neeraj

>   Documentation/RCU/Design/Requirements/Requirements.rst | 18 ++++++++++++++++++
>   1 file changed, 18 insertions(+)
> 
> diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
> index 1e3df77..2dce79d 100644
> --- a/Documentation/RCU/Design/Requirements/Requirements.rst
> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
> @@ -2600,6 +2600,24 @@ also includes DEFINE_SRCU(), DEFINE_STATIC_SRCU(), and
>   init_srcu_struct() APIs for defining and initializing
>   ``srcu_struct`` structures.
>   
> +More recently, the SRCU API has added polling interfaces:
> +
> +#. start_poll_synchronize_srcu() returns a cookie identifying
> +   the completion of a future SRCU grace period and ensures
> +   that this grace period will be started.
> +#. poll_state_synchronize_srcu() returns ``true`` iff the
> +   specified cookie corresponds to an already-completed
> +   SRCU grace period.
> +#. get_state_synchronize_srcu() returns a cookie just like
> +   start_poll_synchronize_srcu() does, but differs in that
> +   it does nothing to ensure that any future SRCU grace period
> +   will be started.
> +
> +These functions are used to avoid unnecessary SRCU grace periods in
> +certain types of buffer-cache algorithms having multi-stage age-out
> +mechanisms.  The idea is that by the time the block has aged completely
> +from the cache, an SRCU grace period will be very likely to have elapsed.
> +
>   Tasks RCU
>   ~~~~~~~~~
>   
> 

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