Re: [PATCH 09/10] xfs: poll waiting for quotacheck

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Tue, Jun 11, 2019 at 11:07:12AM -0400, Brian Foster wrote:
> On Tue, Jun 04, 2019 at 02:50:27PM -0700, Darrick J. Wong wrote:
> > From: Darrick J. Wong <darrick.wong@oracle.com>
> > 
> > Create a pwork destroy function that uses polling instead of
> > uninterruptible sleep to wait for work items to finish so that we can
> > touch the softlockup watchdog.  IOWs, gross hack.
> > 
> > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> 
> Seems reasonable given the quirky circumstances of quotacheck. Just a
> couple nits..
> 
> >  fs/xfs/xfs_iwalk.c |    3 +++
> >  fs/xfs/xfs_iwalk.h |    3 ++-
> >  fs/xfs/xfs_pwork.c |   21 +++++++++++++++++++++
> >  fs/xfs/xfs_pwork.h |    2 ++
> >  fs/xfs/xfs_qm.c    |    2 +-
> >  5 files changed, 29 insertions(+), 2 deletions(-)
> > 
> > 
> > diff --git a/fs/xfs/xfs_iwalk.c b/fs/xfs/xfs_iwalk.c
> > index 71ee1628aa70..c4a9c4c246b7 100644
> > --- a/fs/xfs/xfs_iwalk.c
> > +++ b/fs/xfs/xfs_iwalk.c
> > @@ -526,6 +526,7 @@ xfs_iwalk_threaded(
> >  	xfs_ino_t		startino,
> >  	xfs_iwalk_fn		iwalk_fn,
> >  	unsigned int		max_prefetch,
> > +	bool			polled,
> >  	void			*data)
> >  {
> >  	struct xfs_pwork_ctl	pctl;
> > @@ -556,5 +557,7 @@ xfs_iwalk_threaded(
> >  		startino = XFS_AGINO_TO_INO(mp, agno + 1, 0);
> >  	}
> >  
> > +	if (polled)
> > +		return xfs_pwork_destroy_poll(&pctl);
> >  	return xfs_pwork_destroy(&pctl);
> 
> Rather than have duplicate destruction paths, could we rework
> xfs_pwork_destroy_poll() to something like xfs_pwork_poll() that just
> does the polling and returns, then the caller can fall back into the
> current xfs_pwork_destroy()? I.e., this ends up looking like:
> 
> 	...
> 	/* poll to keep soft lockup watchdog quiet */
> 	if (polled)
> 		xfs_pwork_poll(&pctl);
> 	return xfs_pwork_destroy(&pctl);

Sounds good, will do!

> >  }
> ...
> > diff --git a/fs/xfs/xfs_pwork.c b/fs/xfs/xfs_pwork.c
> > index 19605a3a2482..3b885e0b52ac 100644
> > --- a/fs/xfs/xfs_pwork.c
> > +++ b/fs/xfs/xfs_pwork.c
> ...
> > @@ -97,6 +101,23 @@ xfs_pwork_destroy(
> >  	return pctl->error;
> >  }
> >  
> > +/*
> > + * Wait for the work to finish and tear down the control structure.
> > + * Continually poll completion status and touch the soft lockup watchdog.
> > + * This is for things like mount that hold locks.
> > + */
> > +int
> > +xfs_pwork_destroy_poll(
> > +	struct xfs_pwork_ctl	*pctl)
> > +{
> > +	while (atomic_read(&pctl->nr_work) > 0) {
> > +		msleep(1);
> > +		touch_softlockup_watchdog();
> > +	}
> 
> Any idea what the typical watchdog timeout is?

Usually 30s for the hangcheck timeout.

> I'm curious where the 1ms
> comes from and whether we could get away with anything larger. I realize
> that might introduce mount latency with the current sleep based
> implementation, but we could also consider a waitqueue and using
> something like wait_event_timeout() to schedule out for longer time
> periods and still wake up immediately when the count drops to 0.

That's a much better approach than this naïve one which waits
unnecessarily after the pwork finishes; I'll replace it with this.
The kernel doesn't export the hang check timeout variable, so I think
I'll just set it to 1s, which should be infrequent enough not to use a
lot of CPU and frequent enough that we don't spew warnings everywhere.

--D

> Brian
> 
> > +
> > +	return xfs_pwork_destroy(pctl);
> > +}
> > +
> >  /*
> >   * Return the amount of parallelism that the data device can handle, or 0 for
> >   * no limit.
> > diff --git a/fs/xfs/xfs_pwork.h b/fs/xfs/xfs_pwork.h
> > index e0c1354a2d8c..08da723a8dc9 100644
> > --- a/fs/xfs/xfs_pwork.h
> > +++ b/fs/xfs/xfs_pwork.h
> > @@ -18,6 +18,7 @@ struct xfs_pwork_ctl {
> >  	struct workqueue_struct	*wq;
> >  	struct xfs_mount	*mp;
> >  	xfs_pwork_work_fn	work_fn;
> > +	atomic_t		nr_work;
> >  	int			error;
> >  };
> >  
> > @@ -45,6 +46,7 @@ int xfs_pwork_init(struct xfs_mount *mp, struct xfs_pwork_ctl *pctl,
> >  		unsigned int nr_threads);
> >  void xfs_pwork_queue(struct xfs_pwork_ctl *pctl, struct xfs_pwork *pwork);
> >  int xfs_pwork_destroy(struct xfs_pwork_ctl *pctl);
> > +int xfs_pwork_destroy_poll(struct xfs_pwork_ctl *pctl);
> >  unsigned int xfs_pwork_guess_datadev_parallelism(struct xfs_mount *mp);
> >  
> >  #endif /* __XFS_PWORK_H__ */
> > diff --git a/fs/xfs/xfs_qm.c b/fs/xfs/xfs_qm.c
> > index e4f3785f7a64..de6a623ada02 100644
> > --- a/fs/xfs/xfs_qm.c
> > +++ b/fs/xfs/xfs_qm.c
> > @@ -1305,7 +1305,7 @@ xfs_qm_quotacheck(
> >  		flags |= XFS_PQUOTA_CHKD;
> >  	}
> >  
> > -	error = xfs_iwalk_threaded(mp, 0, xfs_qm_dqusage_adjust, 0, NULL);
> > +	error = xfs_iwalk_threaded(mp, 0, xfs_qm_dqusage_adjust, 0, true, NULL);
> >  	if (error)
> >  		goto error_return;
> >  
> >