Messages in this thread | | | Date | Thu, 12 May 2022 06:13:35 -0700 | From | Dmitry Torokhov <> | Subject | Re: [PATCH v3 (repost)] workqueue: Warn flushing of kernel-global workqueues |
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On Thu, May 12, 2022 at 08:32:10PM +0900, Tetsuo Handa wrote: > On 2022/05/12 19:38, Dmitry Torokhov wrote: > > Hi Tejun, > > > > On Mon, Mar 21, 2022 at 07:02:45AM -1000, Tejun Heo wrote: > >> I'm willing to bet that the majority of the use cases can be converted to > >> use flush_work() and that'd be the preference. We need a separate workqueue > >> iff the flush requrement is complex (e.g. there are multiple dynamic work > >> items in flight which need to be flushed together) or the work items needs > >> some special attributes (such as MEM_RECLAIM or HIGHPRI) which don't apply > >> to the system_wq users in the first place. > > > > This means that now the code has to keep track of all work items that it > > allocated, instead of being able "fire and forget" works (when dealing > > with extremely infrequent events) and rely on flush_workqueue() to > > cleanup. > > Yes. Moreover, a patch to catch and refuse at compile time was proposed at > https://lkml.kernel.org/r/738afe71-2983-05d5-f0fc-d94efbdf7634@I-love.SAKURA.ne.jp .
My comment was not a wholesale endorsement of Tejun's statement, but rather a note of the fact that it again adds complexity (at least as far as driver writers are concerned) to the kernel code.
Also as far as I can see the patch was rejected.
> > > That flush typically happens in module unload path, and I > > wonder if the restriction on flush_workqueue() could be relaxed to allow > > calling it on unload. > > A patch for drivers/input/mouse/psmouse-smbus.c is waiting for your response at > https://lkml.kernel.org/r/25e2b787-cb2c-fb0d-d62c-6577ad1cd9df@I-love.SAKURA.ne.jp . > Like many modules, flush_workqueue() happens on only module unload in your case.
Yes, I saw that patch, and that is what prompted my response. I find it adding complexity and I was wondering if it could be avoided. It also unclear to me if there is an additional cost coming from allocating a dedicated workqueue.
> > We currently don't have a flag to tell whether the caller is inside module unload > path. And even inside module unload path, flushing the system-wide workqueue is > problematic under e.g. GFP_NOFS/GFP_NOIO context.
Sorry, I do not follow here. Are there module unloading code that runs as GFP_NOFS/GFP_NOIO?
> Therefore, I don't think that > the caller is inside module unload path as a good exception. > > Removing flush_scheduled_work() is for proactively avoiding new problems like > https://lkml.kernel.org/r/385ce718-f965-4005-56b6-34922c4533b8@I-love.SAKURA.ne.jp > and https://lkml.kernel.org/r/20220225112405.355599-10-Jerome.Pouiller@silabs.com . > > Using local WQ also helps for documentation purpose. > This change makes clear where the work's dependency is. > Please grep the linux-next.git tree. Some have been already converted.
I understand that for some of them the change makes sense, but it would be nice to continue using simple API under limited circumstances.
> > Any chance you have too many out-of-tree modules to convert? >
No, we are trying to get everything upstream.
Thanks.
-- Dmitry
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