linux-kernel - Re: [tip/core/rcu, 05/21] rcu: Make rcu_gp_cleanup() more accurately predict need for new GP

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Message-ID: <20180510173754.GB228531@joelaf.mtv.corp.google.com>
Date:   Thu, 10 May 2018 10:37:54 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.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.opensrc@...il.com,
        torvalds@...ux-foundation.org, npiggin@...il.com
Subject: Re: [tip/core/rcu, 05/21] rcu: Make rcu_gp_cleanup() more accurately
 predict need for new GP

On Thu, May 10, 2018 at 06:15:46AM -0700, Paul E. McKenney wrote:
[...] 
> > Also in rcu_future_gp_cleanup, we call:
> > 	trace_rcu_future_gp(rnp, rdp, c,
> > 			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));
> > For this case, in the final trace event record, rnp->completed and c will be
> > the same, since c is set to rnp->completed before calling
> > trace_rcu_future_gp. I was thinking they should be different, do you expect
> > them to be the same?
> 
> Hmmm...  That does look a bit inconsistent.  And it currently uses
> rnp->gp_seq instead of rnp->gp_seq_needed despite having the same
> "CleanupMore" name.
> 
> Looks like a review of the calls to trace_rcu_this_gp() is in order.

I see you changed trace_rcu_future_gp to use trace_rcu_this_gp in 15/21.. I
am not sure if the concern is still valid then since you seem to be correctly
getting the future GP in those cases, except for the naming which I suggest
be changed from 'c' to 'future_gp' just for clarity / self-documenting code.

thanks,

- Joel