From: "Paul E. McKenney via lttng-dev" <lttng-dev@lists.lttng.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: yaowenbin <yaowenbin1@huawei.com>,
lttng-dev <lttng-dev@lists.lttng.org>,
wuxu wu <wuxu.wu@huawei.com>, h00486469 <hewenliang4@huawei.com>
Subject: Re: [lttng-dev] [PATCH] QSBR: Use xor operation to replace add operation when changing rcu_gp.ctr value
Date: Wed, 16 Feb 2022 13:12:41 -0800 [thread overview]
Message-ID: <20220216211241.GO4285@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <2033818248.4933.1645044800494.JavaMail.zimbra@efficios.com>
On Wed, Feb 16, 2022 at 03:53:20PM -0500, Mathieu Desnoyers wrote:
> ----- On Feb 16, 2022, at 2:35 AM, lttng-dev lttng-dev@lists.lttng.org wrote:
>
> > It is enough to have three values of rcu_gp.ctr, 00 for INACTIVE,
> > 01 or 11 for ACTIVE. So it is possible to replace add operation
> > with xor operation when changing rcu_gp.ctr value.
>
> What is missing here is a description justifying why this change is useful.
>
> What is inherently better about XOR compared to ADD or even binary-OR ?
>
> If it's about performance, then a benchmark on relevant architectures
> would be useful. But I suspect that if end users care that much about the
> performance of urcu_qsbr_synchronize_rcu(), they might be doing something
> wrong.
Plus having the full counter can be extremely helpful when debugging.
Thanx, Paul
> Thanks,
>
> Mathieu
>
> >
> > Signed-off-by: yaowenbin <yaowenbin1@huawei.com>
> > ---
> > src/urcu-qsbr.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/src/urcu-qsbr.c b/src/urcu-qsbr.c
> > index 3709412..46135f9 100644
> > --- a/src/urcu-qsbr.c
> > +++ b/src/urcu-qsbr.c
> > @@ -391,7 +391,7 @@ void urcu_qsbr_synchronize_rcu(void)
> > goto out;
> >
> > /* Increment current G.P. */
> > - CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr + URCU_QSBR_GP_CTR);
> > + CMM_STORE_SHARED(urcu_qsbr_gp.ctr, urcu_qsbr_gp.ctr ^ URCU_QSBR_GP_CTR);
> >
> > /*
> > * Must commit urcu_qsbr_gp.ctr update to memory before waiting for
> > --
> > 2.27.0
> > _______________________________________________
> > lttng-dev mailing list
> > lttng-dev@lists.lttng.org
> > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com
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prev parent reply other threads:[~2022-02-16 21:22 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-16 7:35 yaowenbin via lttng-dev
2022-02-16 20:53 ` Mathieu Desnoyers via lttng-dev
2022-02-16 21:12 ` Paul E. McKenney via lttng-dev [this message]
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