linux-kernel - Re: RFC: call_rcu_outstanding (was Re: WARNING in _

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Date:   Sun, 21 Jul 2019 14:08:37 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, aarcange@...hat.com,
        akpm@...ux-foundation.org, christian@...uner.io,
        davem@...emloft.net, ebiederm@...ssion.com,
        elena.reshetova@...el.com, guro@...com, hch@...radead.org,
        james.bottomley@...senpartnership.com, jasowang@...hat.com,
        jglisse@...hat.com, keescook@...omium.org, ldv@...linux.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-parisc@...r.kernel.org,
        luto@...capital.net, mhocko@...e.com, mingo@...nel.org,
        namit@...are.com, peterz@...radead.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk,
        wad@...omium.org
Subject: Re: RFC: call_rcu_outstanding (was Re: WARNING in __mmdrop)

On Sun, Jul 21, 2019 at 06:17:25AM -0700, Paul E. McKenney wrote:
> Also, the overhead is important.  For example, as far as I know,
> current RCU gracefully handles close(open(...)) in a tight userspace
> loop.  But there might be trouble due to tight userspace loops around
> lighter-weight operations.

I thought you believed that RCU was antifragile, in that it would scale
better as it was used more heavily?

Would it make sense to have call_rcu() check to see if there are many
outstanding requests on this CPU and if so process them before returning?
That would ensure that frequent callers usually ended up doing their
own processing.