linux-kernel - Re: [PATCH 1/3] kthread: add a mechanism to store cgroup info

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Message-ID: <20170908143537.GW1774378@devbig577.frc2.facebook.com>
Date:   Fri, 8 Sep 2017 07:35:37 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Shaohua Li <shli@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        lizefan@...wei.com, tglx@...utronix.de, kernel-team@...com,
        axboe@...nel.dk, Shaohua Li <shli@...com>
Subject: Re: [PATCH 1/3] kthread: add a mechanism to store cgroup info

Hello,

On Wed, Sep 06, 2017 at 07:00:51PM -0700, Shaohua Li wrote:
> +#ifdef CONFIG_CGROUPS
> +void kthread_set_orig_css(struct cgroup_subsys_state *css);
> +struct cgroup_subsys_state *kthread_get_orig_css(void);
> +void kthread_reset_orig_css(void);

* It's a bit weird to associate a kthread with a css without being
  specific.  If what's needed is a generic association (this kthread
  is temporarily servicing this cgroup), it should be associated with
  the cgroup.  But, I think it'd be better to make it specific instead
  - ie. kthread_set_io_css().

* Do we need the reset function to be separate?  Can't we just clear
  it when the set function is called with NULL css?

* For the accessor, can we do sth like kthread_orig_css() (or
  kthread_io_css())?  "get" is overloaded between set/get and get/put,
  so it can get confusing.

> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index c05ac5f..ab2295d 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1276,6 +1276,7 @@ extern struct pid *cad_pid;
>  #define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
>  #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_allowed */
>  #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
> +#define PF_KTHREAD_HAS_CSS	0x10000000	/* kthread has css info attached */

Do we need a separate flag for this?  kthreads already have PF_KTHREAD
set.  I'm not sure why we'd need another flag.  Once we know it's a
kthread, we can just access its css pointer field, right?

Thanks.

-- 
tejun