linux-kernel - Re: [PATCH] workqueue: Drop the NOT_RUNNING check to flags in worker_{set,clr}

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Date:   Mon, 27 Mar 2023 11:07:39 +0800
From:   Yue Hu <zbestahu@...il.com>
To:     Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, huyue2@...lpad.com,
        zhangwen@...lpad.com
Subject: Re: [PATCH] workqueue: Drop the NOT_RUNNING check to flags in
 worker_{set,clr}_flags

Hi Tejun, Lai,

On Sat, 25 Mar 2023 17:20:14 +0800
Lai Jiangshan <jiangshanlai@...il.com> wrote:

> On Fri, Mar 24, 2023 at 9:58 AM Tejun Heo <tj@...nel.org> wrote:
> >
> > On Mon, Mar 20, 2023 at 05:26:52PM +0800, Yue Hu wrote:  
> > > From: Yue Hu <huyue2@...lpad.com>
> > >
> > > We know whether the worker flags are belong to WORKER_NOT_RUNNING or not
> > > when we are setting and clearing them.  So check the flags not running
> > > related is unnecessary for both the cases.
> > >
> > > Currently, worker_{set,clr}_flags() are all used for WORKER_NOT_RUNNING
> > > except for clearing WORKER_IDLE.  Let's change to directly clear it
> > > instead.  Also, update the comment a little in worker_clr_flags().  
> >
> > I'm not sure this is better. Semantically, the existing code seems clearer
> > and less error-prone to me and this isn't gonna make any meaningful perf
> > difference. Lai, what do you think?  
> 
> objdump -DSr kernel/workqueue.o | less
> 
>     3275:       00 00
>                         3273: R_X86_64_32S      current_task
>         WARN_ON_ONCE(worker->task != current);
>     3277:       48 39 43 40             cmp    %rax,0x40(%rbx)
>     327b:       0f 85 91 00 00 00       jne    3312 <process_one_work+0x3a2>
>             !(worker->flags & WORKER_NOT_RUNNING)) {
>     3281:       8b 43 68                mov    0x68(%rbx),%eax
>         if ((flags & WORKER_NOT_RUNNING) &&
>     3284:       a9 c8 01 00 00          test   $0x1c8,%eax
>     3289:       75 0b                   jne    3296 <process_one_work+0x326>
>         struct worker_pool *pool = worker->pool;
>     328b:       48 8b 43 48             mov    0x48(%rbx),%rax
>                 pool->nr_running--;
>     328f:       83 68 20 01             subl   $0x1,0x20(%rax)
>     3293:       8b 43 68                mov    0x68(%rbx),%eax
>         worker->flags |= flags;
>     3296:       83 c8 40                or     $0x40,%eax
>     3299:       89 43 68                mov    %eax,0x68(%rbx)
> 
> It seems the compiler will do the trick. The clearer existing code
> seems better.

Thank you for taking time to review the patch.

The `worker_set_flags` has two behaviors. One is set worker flags unconditionally.
Another is to conditionally adjust `nr_running`. I understand the adjustment should be
only for the flags that we set to NOT_RUNNING. 

And currently the `worker_set_flags()` is not universally used when setting them, such
as in the case of setting WOKER_IDLE. So, I think this helper is not fulfilling its
responsibility, it is actually just only processing `not_running` state. 

If the change is meaningful, maybe it's better to name it as worker_set_not_running_flags()?

Anyway, it is just a minor change.

Thanks.

> 
> Thanks
> Lai
> 
> >
> > Thanks.
> >
> > --
> > tejun