linux-kernel - Re: RCU: Question on force_qs

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Message-ID: <20200914194208.GA2579423@google.com>
Date:   Mon, 14 Sep 2020 15:42:08 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Zhang, Qiang" <Qiang.Zhang@...driver.com>
Cc:     "paulmck@...nel.org" <paulmck@...nel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        "josh@...htriplett.org" <josh@...htriplett.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mathieu.desnoyers@...icios.com" <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: RCU: Question on force_qs_rnp

On Mon, Sep 14, 2020 at 07:55:18AM +0000, Zhang, Qiang wrote:
> Hello Paul
> 
> I have some questions for you .
> in force_qs_rnp func ,  if  "f(rdp)" func return true we will call rcu_report_qs_rnp func
> report a quiescent state for this rnp node, and clear grpmask form rnp->qsmask.
> after that ,  can we make a check for this rnp->qsmask,  if  rnp->qsmask == 0, 
> we will check blocked readers in this rnp node,  instead of jumping directly to the next node .

Could you clarify what good is this going to do? What problem are you trying to
address?

You could have a task that is blocked in an RCU leaf node, but the
force_qs_rnp() decided to call rcu_report_qs_rnp(). This is perfectly Ok. The
CPU could be dyntick-idle and a quiescent state is reported. However, the GP
must not end and the rcu leaf node should still be present in its parent
intermediate nodes ->qsmask. In this case, the ->qsmask == 0 does not have
any relevance.

Or am I missing the point of the question?

thanks,

 - Joel