[PATCH 1/2 V3] MM: replace PF_LESS_THROTTLE with PF_LOCAL_THROTTLE

From: NeilBrown
Date: Wed Apr 15 2020 - 20:31:14 EST


PF_LESS_THROTTLE exists for loop-back nfsd (and a similar need in the
loop block driver and callers of prctl(PR_SET_IO_FLUSHER)), where a
daemon needs to write to one bdi (the final bdi) in order to free up
writes queued to another bdi (the client bdi).

The daemon sets PF_LESS_THROTTLE and gets a larger allowance of dirty
pages, so that it can still dirty pages after other processses have been
throttled.

This approach was designed when all threads were blocked equally,
independently on which device they were writing to, or how fast it was.
Since that time the writeback algorithm has changed substantially with
different threads getting different allowances based on non-trivial
heuristics. This means the simple "add 25%" heuristic is no longer
reliable.

The important issue is not that the daemon needs a *larger* dirty page
allowance, but that it needs a *private* dirty page allowance, so that
dirty pages for the "client" bdi that it is helping to clear (the bdi for
an NFS filesystem or loop block device etc) do not affect the throttling
of the deamon writing to the "final" bdi.

This patch changes the heuristic so that the task is only throttled if
*both* the global threshhold *and* the per-wb threshold are exceeded.
This is similar to the effect of BDI_CAP_STRICTLIMIT which causes the
global limits to be ignored, but it isn't as strict. A PF_LOCAL_THROTTLE
task will be allowed to proceed unthrottled if the global threshold is
not exceeded or if the local threshold is not exceeded. They need to
both be exceeded before PF_LOCAL_THROTTLE tasks are throttled.

This approach of "only throttle when target bdi is busy" is consistent
with the other use of PF_LESS_THROTTLE in current_may_throttle(), were
it causes attention to be focussed only on the target bdi.

So this patch
- renames PF_LESS_THROTTLE to PF_LOCAL_THROTTLE,
- removes the 25% bonus that that flag gives, and
- If PF_LOCAL_THROTTLE is set, don't delay at all unless both
thresholds are exceeded.

Note that previously realtime threads were treated the same as
PF_LESS_THROTTLE threads. This patch does *not* change the behvaiour for
real-time threads, so it is now different from the behaviour of nfsd and
loop tasks. I don't know what is wanted for realtime.

Acked-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
drivers/block/loop.c | 2 +-
fs/nfsd/vfs.c | 9 +++++----
include/linux/sched.h | 3 ++-
kernel/sys.c | 2 +-
mm/page-writeback.c | 18 ++++++++++++++----
mm/vmscan.c | 4 ++--
6 files changed, 25 insertions(+), 13 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index da693e6a834e..d89c25ba3b89 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -919,7 +919,7 @@ static void loop_unprepare_queue(struct loop_device *lo)

static int loop_kthread_worker_fn(void *worker_ptr)
{
- current->flags |= PF_LESS_THROTTLE | PF_MEMALLOC_NOIO;
+ current->flags |= PF_LOCAL_THROTTLE | PF_MEMALLOC_NOIO;
return kthread_worker_fn(worker_ptr);
}

diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index 0aa02eb18bd3..c3fbab1753ec 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -979,12 +979,13 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,

if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
/*
- * We want less throttling in balance_dirty_pages()
- * and shrink_inactive_list() so that nfs to
+ * We want throttling in balance_dirty_pages()
+ * and shrink_inactive_list() to only consider
+ * the backingdev we are writing to, so that nfs to
* localhost doesn't cause nfsd to lock up due to all
* the client's dirty pages or its congested queue.
*/
- current->flags |= PF_LESS_THROTTLE;
+ current->flags |= PF_LOCAL_THROTTLE;

exp = fhp->fh_export;
use_wgather = (rqstp->rq_vers == 2) && EX_WGATHER(exp);
@@ -1037,7 +1038,7 @@ nfsd_vfs_write(struct svc_rqst *rqstp, struct svc_fh *fhp, struct nfsd_file *nf,
nfserr = nfserrno(host_err);
}
if (test_bit(RQ_LOCAL, &rqstp->rq_flags))
- current_restore_flags(pflags, PF_LESS_THROTTLE);
+ current_restore_flags(pflags, PF_LOCAL_THROTTLE);
return nfserr;
}

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5cb8324..5955a089df32 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1481,7 +1481,8 @@ extern struct pid *cad_pid;
#define PF_KSWAPD 0x00020000 /* I am kswapd */
#define PF_MEMALLOC_NOFS 0x00040000 /* All allocation requests will inherit GFP_NOFS */
#define PF_MEMALLOC_NOIO 0x00080000 /* All allocation requests will inherit GFP_NOIO */
-#define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */
+#define PF_LOCAL_THROTTLE 0x00100000 /* Throttle writes only agasint the bdi I write to,
+ * I am cleaning dirty pages from some other bdi. */
#define PF_KTHREAD 0x00200000 /* I am a kernel thread */
#define PF_RANDOMIZE 0x00400000 /* Randomize virtual address space */
#define PF_SWAPWRITE 0x00800000 /* Allowed to write to swap */
diff --git a/kernel/sys.c b/kernel/sys.c
index d325f3ab624a..180a2fa33f7f 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2262,7 +2262,7 @@ int __weak arch_prctl_spec_ctrl_set(struct task_struct *t, unsigned long which,
return -EINVAL;
}

-#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LESS_THROTTLE)
+#define PR_IO_FLUSHER (PF_MEMALLOC_NOIO | PF_LOCAL_THROTTLE)

SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
unsigned long, arg4, unsigned long, arg5)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 7326b54ab728..9692c553526b 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -387,8 +387,7 @@ static unsigned long global_dirtyable_memory(void)
* Calculate @dtc->thresh and ->bg_thresh considering
* vm_dirty_{bytes|ratio} and dirty_background_{bytes|ratio}. The caller
* must ensure that @dtc->avail is set before calling this function. The
- * dirty limits will be lifted by 1/4 for PF_LESS_THROTTLE (ie. nfsd) and
- * real-time tasks.
+ * dirty limits will be lifted by 1/4 for real-time tasks.
*/
static void domain_dirty_limits(struct dirty_throttle_control *dtc)
{
@@ -436,7 +435,7 @@ static void domain_dirty_limits(struct dirty_throttle_control *dtc)
if (bg_thresh >= thresh)
bg_thresh = thresh / 2;
tsk = current;
- if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk)) {
+ if (rt_task(tsk)) {
bg_thresh += bg_thresh / 4 + global_wb_domain.dirty_limit / 32;
thresh += thresh / 4 + global_wb_domain.dirty_limit / 32;
}
@@ -486,7 +485,7 @@ static unsigned long node_dirty_limit(struct pglist_data *pgdat)
else
dirty = vm_dirty_ratio * node_memory / 100;

- if (tsk->flags & PF_LESS_THROTTLE || rt_task(tsk))
+ if (rt_task(tsk))
dirty += dirty / 4;

return dirty;
@@ -1700,6 +1699,17 @@ static void balance_dirty_pages(struct bdi_writeback *wb,
sdtc = mdtc;
}

+ if (current->flags & PF_LOCAL_THROTTLE)
+ /* This task must only be throttled based on the bdi
+ * it is writing to - dirty pages for other bdis might
+ * be pages this task is trying to write out. So it
+ * gets a free pass unless both global and local
+ * thresholds are exceeded. i.e unless
+ * "dirty_exceeded".
+ */
+ if (!dirty_exceeded)
+ break;
+
if (dirty_exceeded && !wb->dirty_exceeded)
wb->dirty_exceeded = 1;

diff --git a/mm/vmscan.c b/mm/vmscan.c
index b06868fc4926..80ab523926aa 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1879,13 +1879,13 @@ static unsigned noinline_for_stack move_pages_to_lru(struct lruvec *lruvec,

/*
* If a kernel thread (such as nfsd for loop-back mounts) services
- * a backing device by writing to the page cache it sets PF_LESS_THROTTLE.
+ * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
* In that case we should only throttle if the backing device it is
* writing to is congested. In other cases it is safe to throttle.
*/
static int current_may_throttle(void)
{
- return !(current->flags & PF_LESS_THROTTLE) ||
+ return !(current->flags & PF_LOCAL_THROTTLE) ||
current->backing_dev_info == NULL ||
bdi_write_congested(current->backing_dev_info);
}
--
2.26.0

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