linux-kernel - Re: [PATCH v9 4/7] rcu/trace: Add tracing for how segcb list changes

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Message-ID: <20201103151731.GB432431@lothringen>
Date:   Tue, 3 Nov 2020 16:17:31 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Marco Elver <elver@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        "Uladzislau Rezki (Sony)" <urezki@...il.com>, fweisbec@...il.com,
        neeraj.iitr10@...il.com
Subject: Re: [PATCH v9 4/7] rcu/trace: Add tracing for how segcb list changes

On Tue, Nov 03, 2020 at 09:26:00AM -0500, Joel Fernandes (Google) wrote:
> +/*
> + * Return how many CBs each segment along with their gp_seq values.
> + *
> + * This function is O(N) where N is the number of segments. Only used from
> + * tracing code which is usually disabled in production.
> + */
> +#ifdef CONFIG_RCU_TRACE
> +static void rcu_segcblist_countseq(struct rcu_segcblist *rsclp,
> +			 int cbcount[RCU_CBLIST_NSEGS],
> +			 unsigned long gpseq[RCU_CBLIST_NSEGS])
> +{
> +	int i;
> +
> +	for (i = 0; i < RCU_CBLIST_NSEGS; i++) {
> +		cbcount[i] = rcu_segcblist_get_seglen(rsclp, i);
> +		gpseq[i] = rsclp->gp_seq[i];
> +	}
> +}
> +
> +void __trace_rcu_segcb_stats(struct rcu_segcblist *rsclp, const char *context)
> +{
> +	int cbs[RCU_CBLIST_NSEGS];
> +	unsigned long gps[RCU_CBLIST_NSEGS];
> +
> +	if (!trace_rcu_segcb_stats_enabled())
> +		return;

Yes, very good!

Paul just told me that RCU_TRACE can be used in production so that confirms that we
wanted to avoid this loop of 8 iterations when tracing is disabled.

Thanks.