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

Brian Foster
2019-06-11T17:01:30Z

On Tue, Jun 11, 2019 at 09:06:27AM -0700, Darrick J. Wong wrote:
> 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.
>

Ack, that sounds reasonable to me if the timeout itself is 30s or so.

Brian

> --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;
> > >
> > >