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

Akinobu Mita
2019-08-23T16:00:59Z

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.