Messages in this thread | | | Date | Tue, 9 May 2023 16:53:32 +0200 | From | David Sterba <> | Subject | Re: [PATCH 08/13] btrfs: Use alloc_ordered_workqueue() to create ordered workqueues |
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On Mon, May 08, 2023 at 03:50:27PM -1000, Tejun Heo wrote: > BACKGROUND > ========== > > When multiple work items are queued to a workqueue, their execution order > doesn't match the queueing order. They may get executed in any order and > simultaneously. When fully serialized execution - one by one in the queueing > order - is needed, an ordered workqueue should be used which can be created > with alloc_ordered_workqueue(). > > However, alloc_ordered_workqueue() was a later addition. Before it, an > ordered workqueue could be obtained by creating an UNBOUND workqueue with > @max_active==1. This originally was an implementation side-effect which was > broken by 4c16bd327c74 ("workqueue: restore WQ_UNBOUND/max_active==1 to be > ordered"). Because there were users that depended on the ordered execution, > 5c0338c68706 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered") > made workqueue allocation path to implicitly promote UNBOUND workqueues w/ > @max_active==1 to ordered workqueues. > > While this has worked okay, overloading the UNBOUND allocation interface > this way creates other issues. It's difficult to tell whether a given > workqueue actually needs to be ordered and users that legitimately want a > min concurrency level wq unexpectedly gets an ordered one instead. With > planned UNBOUND workqueue updates to improve execution locality and more > prevalence of chiplet designs which can benefit from such improvements, this > isn't a state we wanna be in forever. > > This patch series audits all callsites that create an UNBOUND workqueue w/ > @max_active==1 and converts them to alloc_ordered_workqueue() as necessary. > > WHAT TO LOOK FOR > ================ > > The conversions are from > > alloc_workqueue(WQ_UNBOUND | flags, 1, args..) > > to > > alloc_ordered_workqueue(flags, args...) > > which don't cause any functional changes. If you know that fully ordered > execution is not ncessary, please let me know. I'll drop the conversion and > instead add a comment noting the fact to reduce confusion while conversion > is in progress. > > If you aren't fully sure, it's completely fine to let the conversion > through. The behavior will stay exactly the same and we can always > reconsider later. > > As there are follow-up workqueue core changes, I'd really appreciate if the > patch can be routed through the workqueue tree w/ your acks. Thanks. > > v3: btrfs_alloc_workqueue() excluded again. The concurrency level of a > workqueue allocated through btrfs_alloc_workqueue() may be dynamically > adjusted through thresh_exec_hook(), so they can't be ordered. > > v2: btrfs_alloc_workqueue() updated too as suggested by Wang. > > Signed-off-by: Tejun Heo <tj@kernel.org> > Cc: Wang Yugui <wangyugui@e16-tech.com> > Cc: Chris Mason <clm@fb.com> > Cc: Josef Bacik <josef@toxicpanda.com> > Cc: David Sterba <dsterba@suse.com> > Cc: linux-btrfs@vger.kernel.org > --- > fs/btrfs/disk-io.c | 2 +- > fs/btrfs/scrub.c | 6 ++++-- > 2 files changed, 5 insertions(+), 3 deletions(-) > > diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c > index 59ea049fe7ee..32d08aed88b6 100644 > --- a/fs/btrfs/disk-io.c > +++ b/fs/btrfs/disk-io.c > @@ -2217,7 +2217,7 @@ static int btrfs_init_workqueues(struct btrfs_fs_info *fs_info) > fs_info->qgroup_rescan_workers = > btrfs_alloc_workqueue(fs_info, "qgroup-rescan", flags, 1, 0); > fs_info->discard_ctl.discard_workers = > - alloc_workqueue("btrfs_discard", WQ_UNBOUND | WQ_FREEZABLE, 1); > + alloc_ordered_workqueue("btrfs_discard", WQ_FREEZABLE); > > if (!(fs_info->workers && fs_info->hipri_workers && > fs_info->delalloc_workers && fs_info->flush_workers &&
I think there are a few more conversions missing. There's a local flags variable in btrfs_init_workqueues
2175 static int btrfs_init_workqueues(struct btrfs_fs_info *fs_info) 2176 { 2177 u32 max_active = fs_info->thread_pool_size; 2178 unsigned int flags = WQ_MEM_RECLAIM | WQ_FREEZABLE | WQ_UNBOUND;
And used like
2194 fs_info->fixup_workers = 2195 btrfs_alloc_workqueue(fs_info, "fixup", flags, 1, 0);
2213 fs_info->qgroup_rescan_workers = 2214 btrfs_alloc_workqueue(fs_info, "qgroup-rescan", flags, 1, 0);
WQ_UNBOUND is not mentioned explicitliy like for the "btrfs_discard" workqueue. Patch v2 did the switch in btrfs_alloc_workqueue according to the max_active/limit_active parameter but this would affect all queues and not all of them require to be ordered.
In btrfs_resize_thread_pool the workqueue_set_max_active is called directly or indirectly so this can set the max_active to a user-defined mount option. Could this be a problem or trigger a warning? This would lead to max_active==1 + WQ_UNBOUND.
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