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

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Date:   Sat, 25 Mar 2023 17:20:14 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Yue Hu <zbestahu@...il.com>, 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

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.

Thanks
Lai

>
> Thanks.
>
> --
> tejun