Re: [PATCH 08/10] xfs: multithreaded iwalk implementation

[Brian Foster <bfoster@redhat.com>]

On Tue, Jun 04, 2019 at 02:50:20PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
> 
> Create a parallel iwalk implementation and switch quotacheck to use it.
> 
> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---

Interesting.. is there any commonality here with the ktask mechanism
that's been in progress? I've not followed the details, but I thought it
was a similar idea. The last post I see for that is here:

https://marc.info/?l=linux-mm&m=154143701122927&w=2

That aside, this all looks mostly fine to me. A few random thoughts..

>  fs/xfs/Makefile      |    1 
>  fs/xfs/xfs_globals.c |    3 +
>  fs/xfs/xfs_iwalk.c   |   76 ++++++++++++++++++++++++++++++-
>  fs/xfs/xfs_iwalk.h   |    2 +
>  fs/xfs/xfs_pwork.c   |  122 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_pwork.h   |   50 ++++++++++++++++++++
>  fs/xfs/xfs_qm.c      |    2 -
>  fs/xfs/xfs_sysctl.h  |    6 ++
>  fs/xfs/xfs_sysfs.c   |   40 ++++++++++++++++
>  fs/xfs/xfs_trace.h   |   18 +++++++
>  10 files changed, 317 insertions(+), 3 deletions(-)
>  create mode 100644 fs/xfs/xfs_pwork.c
>  create mode 100644 fs/xfs/xfs_pwork.h
> 
> 
> diff --git a/fs/xfs/Makefile b/fs/xfs/Makefile
> index 74d30ef0dbce..48940a27d4aa 100644
> --- a/fs/xfs/Makefile
> +++ b/fs/xfs/Makefile
> @@ -84,6 +84,7 @@ xfs-y				+= xfs_aops.o \
>  				   xfs_message.o \
>  				   xfs_mount.o \
>  				   xfs_mru_cache.o \
> +				   xfs_pwork.o \
>  				   xfs_reflink.o \
>  				   xfs_stats.o \
>  				   xfs_super.o \
> diff --git a/fs/xfs/xfs_globals.c b/fs/xfs/xfs_globals.c
> index d0d377384120..4f93f2c4dc38 100644
> --- a/fs/xfs/xfs_globals.c
> +++ b/fs/xfs/xfs_globals.c
> @@ -31,6 +31,9 @@ xfs_param_t xfs_params = {
>  	.fstrm_timer	= {	1,		30*100,		3600*100},
>  	.eofb_timer	= {	1,		300,		3600*24},
>  	.cowb_timer	= {	1,		1800,		3600*24},
> +#ifdef DEBUG
> +	.pwork_threads	= {	0,		0,		NR_CPUS	},
> +#endif
>  };
>  
>  struct xfs_globals xfs_globals = {
> diff --git a/fs/xfs/xfs_iwalk.c b/fs/xfs/xfs_iwalk.c
> index 8595258b5001..71ee1628aa70 100644
> --- a/fs/xfs/xfs_iwalk.c
> +++ b/fs/xfs/xfs_iwalk.c
> @@ -21,6 +21,7 @@
>  #include "xfs_health.h"
>  #include "xfs_trans.h"
>  #include "xfs_iwalk.h"
> +#include "xfs_pwork.h"
>  
>  /*
>   * Walking Inodes in the Filesystem
> @@ -46,6 +47,9 @@
>   */
>  
>  struct xfs_iwalk_ag {
> +	/* parallel work control data; will be null if single threaded */
> +	struct xfs_pwork		pwork;
> +
>  	struct xfs_mount		*mp;
>  	struct xfs_trans		*tp;
>  
> @@ -200,6 +204,9 @@ xfs_iwalk_ag_recs(
>  		trace_xfs_iwalk_ag_rec(mp, agno, irec);
>  
>  		for (j = 0; j < XFS_INODES_PER_CHUNK; j++) {
> +			if (xfs_pwork_want_abort(&iwag->pwork))
> +				return 0;
> +
>  			/* Skip if this inode is free */
>  			if (XFS_INOBT_MASK(j) & irec->ir_free)
>  				continue;
> @@ -360,7 +367,7 @@ xfs_iwalk_ag(
>  	agino = XFS_INO_TO_AGINO(mp, iwag->startino);
>  	error = xfs_iwalk_ag_start(iwag, agno, agino, &cur, &agi_bp, &has_more);
>  
> -	while (!error && has_more) {
> +	while (!error && has_more && !xfs_pwork_want_abort(&iwag->pwork)) {
>  		struct xfs_inobt_rec_incore	*irec;
>  
>  		cond_resched();
> @@ -409,7 +416,7 @@ xfs_iwalk_ag(
>  	xfs_iwalk_del_inobt(tp, &cur, &agi_bp, error);
>  
>  	/* Walk any records left behind in the cache. */
> -	if (iwag->nr_recs == 0 || error)
> +	if (iwag->nr_recs == 0 || error || xfs_pwork_want_abort(&iwag->pwork))
>  		return error;
>  
>  	return xfs_iwalk_ag_recs(iwag);
> @@ -465,6 +472,7 @@ xfs_iwalk(
>  		.iwalk_fn	= iwalk_fn,
>  		.data		= data,
>  		.startino	= startino,
> +		.pwork		= XFS_PWORK_SINGLE_THREADED,
>  	};
>  	xfs_agnumber_t		agno = XFS_INO_TO_AGNO(mp, startino);
>  	int			error;
> @@ -486,3 +494,67 @@ xfs_iwalk(
>  	xfs_iwalk_free(&iwag);
>  	return error;
>  }
> +
> +/* Run per-thread iwalk work. */
> +static int
> +xfs_iwalk_ag_work(
> +	struct xfs_mount	*mp,
> +	struct xfs_pwork	*pwork)
> +{
> +	struct xfs_iwalk_ag	*iwag;
> +	int			error;
> +
> +	iwag = container_of(pwork, struct xfs_iwalk_ag, pwork);
> +	error = xfs_iwalk_alloc(iwag);
> +	if (error)
> +		goto out;

In most cases this will never fail, but the error path if it does looks
slightly painful. I was thinking if we could move this up into
xfs_iwalk_threaded() so we wouldn't continue to queue work jobs when
failure is imminent...

> +
> +	error = xfs_iwalk_ag(iwag);
> +	xfs_iwalk_free(iwag);
> +out:
> +	kmem_free(iwag);
> +	return error;
> +}
> +
> +/*
> + * Walk all the inodes in the filesystem using multiple threads to process each
> + * AG.
> + */
> +int
> +xfs_iwalk_threaded(
> +	struct xfs_mount	*mp,
> +	xfs_ino_t		startino,
> +	xfs_iwalk_fn		iwalk_fn,
> +	unsigned int		max_prefetch,
> +	void			*data)
> +{
> +	struct xfs_pwork_ctl	pctl;
> +	xfs_agnumber_t		agno = XFS_INO_TO_AGNO(mp, startino);
> +	unsigned int		nr_threads;
> +	int			error;
> +
> +	ASSERT(agno < mp->m_sb.sb_agcount);
> +
> +	nr_threads = xfs_pwork_guess_datadev_parallelism(mp);
> +	error = xfs_pwork_init(mp, &pctl, xfs_iwalk_ag_work, "xfs_iwalk",
> +			nr_threads);
> +	if (error)
> +		return error;
> +
> +	for (; agno < mp->m_sb.sb_agcount; agno++) {
> +		struct xfs_iwalk_ag	*iwag;
> +
> +		iwag = kmem_alloc(sizeof(struct xfs_iwalk_ag), KM_SLEEP);
> +		iwag->mp = mp;
> +		iwag->tp = NULL;
> +		iwag->iwalk_fn = iwalk_fn;
> +		iwag->data = data;
> +		iwag->startino = startino;
> +		iwag->recs = NULL;
> +		xfs_iwalk_set_prefetch(iwag, max_prefetch);
> +		xfs_pwork_queue(&pctl, &iwag->pwork);
> +		startino = XFS_AGINO_TO_INO(mp, agno + 1, 0);
> +	}

... but this is only bound by the number of AGs and so could result in a
large number of allocations. FWIW, I wouldn't expect that to be a
problem in the common case. I'm more thinking about the case of a
specially crafted filesystem designed to cause problems on mount.

> +
> +	return xfs_pwork_destroy(&pctl);
> +}
> diff --git a/fs/xfs/xfs_iwalk.h b/fs/xfs/xfs_iwalk.h
> index 45b1baabcd2d..40233a05a766 100644
> --- a/fs/xfs/xfs_iwalk.h
> +++ b/fs/xfs/xfs_iwalk.h
> @@ -14,5 +14,7 @@ typedef int (*xfs_iwalk_fn)(struct xfs_mount *mp, struct xfs_trans *tp,
>  
>  int xfs_iwalk(struct xfs_mount *mp, struct xfs_trans *tp, xfs_ino_t startino,
>  		xfs_iwalk_fn iwalk_fn, unsigned int max_prefetch, void *data);
> +int xfs_iwalk_threaded(struct xfs_mount *mp, xfs_ino_t startino,
> +		xfs_iwalk_fn iwalk_fn, unsigned int max_prefetch, void *data);
>  
>  #endif /* __XFS_IWALK_H__ */
> diff --git a/fs/xfs/xfs_pwork.c b/fs/xfs/xfs_pwork.c
> new file mode 100644
> index 000000000000..19605a3a2482
> --- /dev/null
> +++ b/fs/xfs/xfs_pwork.c
> @@ -0,0 +1,122 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Copyright (C) 2019 Oracle.  All Rights Reserved.
> + * Author: Darrick J. Wong <darrick.wong@oracle.com>
> + */
> +#include "xfs.h"
> +#include "xfs_fs.h"
> +#include "xfs_shared.h"
> +#include "xfs_format.h"
> +#include "xfs_log_format.h"
> +#include "xfs_trans_resv.h"
> +#include "xfs_mount.h"
> +#include "xfs_trace.h"
> +#include "xfs_sysctl.h"
> +#include "xfs_pwork.h"
> +
> +/*
> + * Parallel Work Queue
> + * ===================
> + *
> + * Abstract away the details of running a large and "obviously" parallelizable
> + * task across multiple CPUs.  Callers initialize the pwork control object with
> + * a desired level of parallelization and a work function.  Next, they embed
> + * struct xfs_pwork in whatever structure they use to pass work context to a
> + * worker thread and queue that pwork.  The work function will be passed the
> + * pwork item when it is run (from process context) and any returned error will
> + * cause all threads to abort.
> + * 

FYI minor whitespace damage (trailing space) on the line above.

> + * This is the rough equivalent of the xfsprogs workqueue code, though we can't
> + * reuse that name here.
> + */
> +
> +/* Invoke our caller's function. */
> +static void
> +xfs_pwork_work(
> +	struct work_struct	*work)
> +{
> +	struct xfs_pwork	*pwork;
> +	struct xfs_pwork_ctl	*pctl;
> +	int			error;
> +
> +	pwork = container_of(work, struct xfs_pwork, work);
> +	pctl = pwork->pctl;
> +	error = pctl->work_fn(pctl->mp, pwork);
> +	if (error && !pctl->error)
> +		pctl->error = error;
> +}
> +
> +/*
> + * Set up control data for parallel work.  @work_fn is the function that will
> + * be called.  @tag will be written into the kernel threads.  @nr_threads is
> + * the level of parallelism desired, or 0 for no limit.
> + */
> +int
> +xfs_pwork_init(
> +	struct xfs_mount	*mp,
> +	struct xfs_pwork_ctl	*pctl,
> +	xfs_pwork_work_fn	work_fn,
> +	const char		*tag,
> +	unsigned int		nr_threads)
> +{
> +#ifdef DEBUG
> +	if (xfs_globals.pwork_threads > 0)
> +		nr_threads = xfs_globals.pwork_threads;
> +#endif

Hmm, it might be useful to have the ability to force the no limit case
from the debug knob. Can we use -1 or something here for "disabled?"

> +	trace_xfs_pwork_init(mp, nr_threads, current->pid);
> +
> +	pctl->wq = alloc_workqueue("%s-%d", WQ_FREEZABLE, nr_threads, tag,
> +			current->pid);
> +	if (!pctl->wq)
> +		return -ENOMEM;
> +	pctl->work_fn = work_fn;
> +	pctl->error = 0;
> +	pctl->mp = mp;
> +
> +	return 0;
> +}
> +
> +/* Queue some parallel work. */
> +void
> +xfs_pwork_queue(
> +	struct xfs_pwork_ctl	*pctl,
> +	struct xfs_pwork	*pwork)
> +{
> +	INIT_WORK(&pwork->work, xfs_pwork_work);
> +	pwork->pctl = pctl;
> +	queue_work(pctl->wq, &pwork->work);
> +}
> +
> +/* Wait for the work to finish and tear down the control structure. */
> +int
> +xfs_pwork_destroy(
> +	struct xfs_pwork_ctl	*pctl)
> +{
> +	destroy_workqueue(pctl->wq);
> +	pctl->wq = NULL;
> +	return pctl->error;
> +}
> +
> +/*
> + * Return the amount of parallelism that the data device can handle, or 0 for
> + * no limit.
> + */
> +unsigned int
> +xfs_pwork_guess_datadev_parallelism(
> +	struct xfs_mount	*mp)
> +{
> +	struct xfs_buftarg	*btp = mp->m_ddev_targp;
> +	int			iomin;
> +	int			ioopt;
> +
> +	if (blk_queue_nonrot(btp->bt_bdev->bd_queue))
> +		return num_online_cpus();
> +	if (mp->m_sb.sb_width && mp->m_sb.sb_unit)
> +		return mp->m_sb.sb_width / mp->m_sb.sb_unit;
> +	iomin = bdev_io_min(btp->bt_bdev);
> +	ioopt = bdev_io_opt(btp->bt_bdev);
> +	if (iomin && ioopt)
> +		return ioopt / iomin;
> +
> +	return 1;

Have you collected any performance data related to these heuristics? I
assume the feature is generally a win, but this also seems like we have
a large window of variance here. E.g., an SSD on a server with hundreds
of CPUs will enable as many threads as CPUs, but a single xTB spindle on
the same box may run single threaded (a quick check of a few local
devices all return an optimal I/O size of 0). Is there really no benefit
parallelizing some of that work in the spinning rust case? What about in
the other direction where we might have a ton of threads for inodes
across AGs that all happen to be in the same project quota, for example?

Brian

> +}
> diff --git a/fs/xfs/xfs_pwork.h b/fs/xfs/xfs_pwork.h
> new file mode 100644
> index 000000000000..e0c1354a2d8c
> --- /dev/null
> +++ b/fs/xfs/xfs_pwork.h
> @@ -0,0 +1,50 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * Copyright (C) 2019 Oracle.  All Rights Reserved.
> + * Author: Darrick J. Wong <darrick.wong@oracle.com>
> + */
> +#ifndef __XFS_PWORK_H__
> +#define __XFS_PWORK_H__
> +
> +struct xfs_pwork;
> +struct xfs_mount;
> +
> +typedef int (*xfs_pwork_work_fn)(struct xfs_mount *mp, struct xfs_pwork *pwork);
> +
> +/*
> + * Parallel work coordination structure.
> + */
> +struct xfs_pwork_ctl {
> +	struct workqueue_struct	*wq;
> +	struct xfs_mount	*mp;
> +	xfs_pwork_work_fn	work_fn;
> +	int			error;
> +};
> +
> +/*
> + * Embed this parallel work control item inside your own work structure,
> + * then queue work with it.
> + */
> +struct xfs_pwork {
> +	struct work_struct	work;
> +	struct xfs_pwork_ctl	*pctl;
> +};
> +
> +#define XFS_PWORK_SINGLE_THREADED	{ .pctl = NULL }
> +
> +/* Have we been told to abort? */
> +static inline bool
> +xfs_pwork_want_abort(
> +	struct xfs_pwork	*pwork)
> +{
> +	return pwork->pctl && pwork->pctl->error;
> +}
> +
> +int xfs_pwork_init(struct xfs_mount *mp, struct xfs_pwork_ctl *pctl,
> +		xfs_pwork_work_fn work_fn, const char *tag,
> +		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);
> +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 a5b2260406a8..e4f3785f7a64 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(mp, NULL, 0, xfs_qm_dqusage_adjust, 0, NULL);
> +	error = xfs_iwalk_threaded(mp, 0, xfs_qm_dqusage_adjust, 0, NULL);
>  	if (error)
>  		goto error_return;
>  
> diff --git a/fs/xfs/xfs_sysctl.h b/fs/xfs/xfs_sysctl.h
> index ad7f9be13087..b555e045e2f4 100644
> --- a/fs/xfs/xfs_sysctl.h
> +++ b/fs/xfs/xfs_sysctl.h
> @@ -37,6 +37,9 @@ typedef struct xfs_param {
>  	xfs_sysctl_val_t fstrm_timer;	/* Filestream dir-AG assoc'n timeout. */
>  	xfs_sysctl_val_t eofb_timer;	/* Interval between eofb scan wakeups */
>  	xfs_sysctl_val_t cowb_timer;	/* Interval between cowb scan wakeups */
> +#ifdef DEBUG
> +	xfs_sysctl_val_t pwork_threads;	/* Parallel workqueue thread count */
> +#endif
>  } xfs_param_t;
>  
>  /*
> @@ -82,6 +85,9 @@ enum {
>  extern xfs_param_t	xfs_params;
>  
>  struct xfs_globals {
> +#ifdef DEBUG
> +	int	pwork_threads;		/* parallel workqueue threads */
> +#endif
>  	int	log_recovery_delay;	/* log recovery delay (secs) */
>  	int	mount_delay;		/* mount setup delay (secs) */
>  	bool	bug_on_assert;		/* BUG() the kernel on assert failure */
> diff --git a/fs/xfs/xfs_sysfs.c b/fs/xfs/xfs_sysfs.c
> index cabda13f3c64..910e6b9cb1a7 100644
> --- a/fs/xfs/xfs_sysfs.c
> +++ b/fs/xfs/xfs_sysfs.c
> @@ -206,11 +206,51 @@ always_cow_show(
>  }
>  XFS_SYSFS_ATTR_RW(always_cow);
>  
> +#ifdef DEBUG
> +/*
> + * Override how many threads the parallel work queue is allowed to create.
> + * This has to be a debug-only global (instead of an errortag) because one of
> + * the main users of parallel workqueues is mount time quotacheck.
> + */
> +STATIC ssize_t
> +pwork_threads_store(
> +	struct kobject	*kobject,
> +	const char	*buf,
> +	size_t		count)
> +{
> +	int		ret;
> +	int		val;
> +
> +	ret = kstrtoint(buf, 0, &val);
> +	if (ret)
> +		return ret;
> +
> +	if (val < 0 || val > NR_CPUS)
> +		return -EINVAL;
> +
> +	xfs_globals.pwork_threads = val;
> +
> +	return count;
> +}
> +
> +STATIC ssize_t
> +pwork_threads_show(
> +	struct kobject	*kobject,
> +	char		*buf)
> +{
> +	return snprintf(buf, PAGE_SIZE, "%d\n", xfs_globals.pwork_threads);
> +}
> +XFS_SYSFS_ATTR_RW(pwork_threads);
> +#endif /* DEBUG */
> +
>  static struct attribute *xfs_dbg_attrs[] = {
>  	ATTR_LIST(bug_on_assert),
>  	ATTR_LIST(log_recovery_delay),
>  	ATTR_LIST(mount_delay),
>  	ATTR_LIST(always_cow),
> +#ifdef DEBUG
> +	ATTR_LIST(pwork_threads),
> +#endif
>  	NULL,
>  };
>  
> diff --git a/fs/xfs/xfs_trace.h b/fs/xfs/xfs_trace.h
> index f9bb1d50bc0e..658cbade1998 100644
> --- a/fs/xfs/xfs_trace.h
> +++ b/fs/xfs/xfs_trace.h
> @@ -3556,6 +3556,24 @@ TRACE_EVENT(xfs_iwalk_ag_rec,
>  		  __entry->startino, __entry->freemask)
>  )
>  
> +TRACE_EVENT(xfs_pwork_init,
> +	TP_PROTO(struct xfs_mount *mp, unsigned int nr_threads, pid_t pid),
> +	TP_ARGS(mp, nr_threads, pid),
> +	TP_STRUCT__entry(
> +		__field(dev_t, dev)
> +		__field(unsigned int, nr_threads)
> +		__field(pid_t, pid)
> +	),
> +	TP_fast_assign(
> +		__entry->dev = mp->m_super->s_dev;
> +		__entry->nr_threads = nr_threads;
> +		__entry->pid = pid;
> +	),
> +	TP_printk("dev %d:%d nr_threads %u pid %u",
> +		  MAJOR(__entry->dev), MINOR(__entry->dev),
> +		  __entry->nr_threads, __entry->pid)
> +)
> +
>  #endif /* _TRACE_XFS_H */
>  
>  #undef TRACE_INCLUDE_PATH
>