Re: [PATCH v4 4/5] block: introduce LED block device activity trigger

Akinobu Mita
2019-08-27T14:03:58Z

2019年8月25日(日) 0:53 Jacek Anaszewski <jacek.anaszewski@gmail.com>:
>
> On 8/23/19 6:00 PM, Akinobu Mita wrote:
> > 2019年8月20日(火) 3:38 Jacek Anaszewski <jacek.anaszewski@gmail.com>:
> >>
> >> On 8/19/19 8:22 PM, Jacek Anaszewski wrote:
> >>> On 8/19/19 4:38 PM, Pavel Machek wrote:
> >>>> On Sat 2019-08-17 22:07:43, Jacek Anaszewski wrote:
> >>>>> On 8/17/19 4:55 PM, Pavel Machek wrote:
> >>>>>> On Fri 2019-08-16 01:59:58, Akinobu Mita wrote:
> >>>>>>> This allows LEDs to be controlled by block device activity.
> >>>>>>>
> >>>>>>> We already have ledtrig-disk (LED disk activity trigger), but the lower
> >>>>>>> level disk drivers need to utilize ledtrig_disk_activity() to make the
> >>>>>>> LED blink.
> >>>>>>>
> >>>>>>> The LED block device trigger doesn't require the lower level drivers to
> >>>>>>> have any instrumentation. The activity is collected by polling the disk
> >>>>>>> stats.
> >>>>>>>
> >>>>>>> Example:
> >>>>>>>
> >>>>>>> echo block-nvme0n1 > /sys/class/leds/diy/trigger
> >>>>>>
> >>>>>> Lets use one trigger "block" and have the device as a parameter,
> >>>>>> please.
> >>>>>>
> >>>>>> We already have 1000 cpu triggers on 1000 cpu machines, and yes, its a
> >>>>>> disaster we'll need to fix. Lets not repeat the same mistake here.
> >>>>>>
> >>>>>> I guess it may be slightly more work. Sorry about that.
> >>>>>
> >>>>> We should be able to list available block devices to set,
> >>>>> so the problem would be not avoided anyway.
> >>>>
> >>>> Should we? We need to list triggers, but we may not list all the devices...
> >>>
> >>> This is similar to usbport trigger that lists available
> >>> ports as files in a sub-directory. We might eventually go
> >>> in this direction.
> >>
> >> I must withdraw this statement. This is not similar to usbport
> >> trigger. The difference is that with ledtrig-block we have separate
> >> triggers per each device and I am not aware if there is some centralized
> >> mechanism similar to blocking_notifier_chain (usb_notifier_list
> >> in drivers/usb/core/notify.c) available for block devices, that
> >> would allow to gather all available block devs under common trigger.
> >>
> >> Moreover I remember Greg once discouraged using notifier chains
> >> as they are unsafe, so we would need some other solution anyway.
> >
> > I start thinking that we should implement the LED block device activity
> > trigger in userspace. The userspace application firstly activates
> > one-shot LED trigger and periodically reads /sys/block/<disk>/stat and
> > writes /sys/class/leds/<led>/shot if there is any disk activity.
>
> This would suboptimal solution. I have another idea - let's get back
> to the implementation of ledtrig-blk in genhd.c. We would be registering
> one trigger on module initialization in a function with __init modifier.
> Then we would need to add/remove triggers to the ledtrig-blk in
> register_blkdev()/unregister_blkdev(). And registered triggers would
> be listed in block_devs directory created by the trigger.
>
> You can compare how drivers/usb/core/ledtrig-usbport.c maintains
> similar directory of usb ports.

It could be possible, but I have yet another idea. What about introducing
/proc/led-triggers and /sys/class/leds/<led>/current-trigger?
The /sys/class/leds/<led>/trigger will be obsoleted by these two files.

The /proc/led-triggers is read only and no PAGE_SIZE limitation by the
seq_file interface. So we can list all triggers in this file.

The /sys/class/leds/<led>/current-trigger is almost identical to
/sys/class/leds/<led>/trigger. The only difference is that
'current-trigger' only shows the current trigger name.