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Message-ID: <1516730480.6999.2.camel@gmx.de>
Subject: Re: [RFC PATCH 0/4] softirq: Per vector threading v3
From:   Mike Galbraith <efault@gmx.de>
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Paolo Abeni <pabeni@redhat.com>
Cc:     David Miller <davem@davemloft.net>,
        Frederic Weisbecker <frederic@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Sasha Levin <alexander.levin@verizon.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Mauro Carvalho Chehab <mchehab@s-opensource.com>,
        Hannes Frederic Sowa <hannes@stressinduktion.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        Dmitry Safonov <dima@arista.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Radu Rendec <rrendec@arista.com>,
        Ingo Molnar <mingo@kernel.org>,
        Stanislaw Gruszka <sgruszka@redhat.com>,
        Rik van Riel <riel@redhat.com>,
        Eric Dumazet <edumazet@google.com>
Date:   Tue, 23 Jan 2018 19:01:20 +0100
In-Reply-To: <CA+55aFwEMe=jV7pvV2dWFKrQ--mAw3buXhdJ-Qk2u-290rTQQQ@mail.gmail.com>
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On Tue, 2018-01-23 at 09:42 -0800, Linus Torvalds wrote:
> On Tue, Jan 23, 2018 at 8:57 AM, Paolo Abeni <pabeni@redhat.com> wrote:
> >
> >> Or is it that the workqueue execution is simply not yielding for some
> >> reason?
> >
> > It's like that.
> >
> > I spent little time on it, so I haven't many data point. I'll try to
> > investigate the scenario later this week.
> 
> Hmm. workqueues seem to use cond_resched_rcu_qs(), which does a
> cond_resched() (and a RCU quiescent note).
> 
> But I wonder if the test triggers the "lets run lots of workqueue
> threads", and then the single-threaded user space just gets blown out
> of the water by many kernel threads. Each thread gets its own "fair"
> amount of CPU, but..

If folks aren't careful with workqueues, they can be a generic
starvation problem.  Like the below in the here and now.

fs/nfs: Add a resched point to nfs_commit_release_pages()

During heavy NFS write, kworkers can do very large amounts of work
without scheduling (82ms traced).  Add a resched point.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Suggested-by: Trond Myklebust <trondmy@primarydata.com>
---
 fs/nfs/write.c |    1 +
 1 file changed, 1 insertion(+)

--- a/fs/nfs/write.c
+++ b/fs/nfs/write.c
@@ -1837,6 +1837,7 @@ static void nfs_commit_release_pages(str
 		set_bit(NFS_CONTEXT_RESEND_WRITES, &req->wb_context->flags);
 	next:
 		nfs_unlock_and_release_request(req);
+		cond_resched();
 	}
 	nfss = NFS_SERVER(data->inode);
 	if (atomic_long_read(&nfss->writeback) < NFS_CONGESTION_OFF_THRESH)