linux-kernel - Re: [PATCH 04/14] init: deps: order network interfaces by link order

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Date:	Mon, 19 Oct 2015 13:31:58 +0200
From:	Alexander Holler <holler@...oftware.de>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Russell King <linux@....linux.org.uk>,
	Grant Likely <grant.likely@...aro.org>
Subject: Re: [PATCH 04/14] init: deps: order network interfaces by link order

Am 19.10.2015 um 12:57 schrieb Alexander Holler:
> Am 18.10.2015 um 12:11 schrieb Alexander Holler:
>> Am 18.10.2015 um 07:59 schrieb Greg Kroah-Hartman:
>>> On Sun, Oct 18, 2015 at 07:20:34AM +0200, Alexander Holler wrote:
>>>> Am 18.10.2015 um 07:14 schrieb Greg Kroah-Hartman:
>>>>> On Sun, Oct 18, 2015 at 06:59:22AM +0200, Alexander Holler wrote:
>>>>>> Am 17.10.2015 um 21:36 schrieb Greg Kroah-Hartman:
>>>>>>
>>>>>>> Again, parallelizing does not solve anything, and causes more
>>>>>>> problems
>>>>>>> _and_ makes things take longer.  Try it, we have done it in the
>>>>>>> past and
>>>>>>> proven this, it's pretty easy to test :)
>>>>>>
>>>>>> Just because I'm curious, may I ask how I would test that in the
>>>>>> easy way
>>>>>> you have in mind? I've just posted the results of my tests (the patch
>>>>>> series) but I wonder what you do have in mind.
>>>>>
>>>>> Use the tool, scripts/bootgraph.pl to create a boot graph of your boot
>>>>> sequence.  That should show you the drivers, or other areas, that are
>>>>> causing your boot to be "slow".
>>>>
>>>> So I've misunderstood you. I've read your paragraph as that it's
>>>> easy to
>>>> test parallelizing.
>>>
>>> Ah, ok, if you want to parallelize everything, add some logic in the
>>> driver core where the probe() callback is made to spin that off into a
>>> new thread for every call, and when it's done, clean up the thread.
>>> That's what I did many years ago to try this all out, if you dig in the
>>> lkml archives there's probably a patch somewhere that you can base the
>>> work off of to test it yourself.
>>
>> Hmm, I don't think I will do that because that means to setup a new
>> thread for every call. And it doesn't need much imagination (or
>> experience) that this introduces quite some overhead.
>>
>> But maybe it makes sense to try out what I'm doing in my patches,
>> starting multiple threads once and then just giving them some work. Will
>
> After a having second thought on your simple approach to parallelize
> stuff, I have to say that it just can't work because just starting a
> thread for every probe() totally ignores possible dependencies.
> Regardless if using one thread per probe() call or if feeding probe()
> calls to just a few threads.
>
> Maybe that's why previous attempts to parallelize stuff failed. But
> that's just an assumption as I'm unaware of these previous attempts.

Or to describe it more verbose, if DEBUG is turned on in 
init/dependencies.c (using my patches), it spits out a summary of groups 
with initcalls (probe() calls) which are independent from each other and 
therfore can be called in parallel. E.g. one of my systems this looks so:

[    0.288229] init: vertices: 429 edges 204 count 170
[    0.288295] init: group 0 length 66 (start 0)
[    0.288329] init: group 1 length 33 (start 66)
[    0.288364] init: group 2 length 13 (start 99)
[    0.288398] init: group 3 length 7 (start 112)
[    0.288432] init: group 4 length 9 (start 119)
[    0.288466] init: group 5 length 8 (start 128)
[    0.288500] init: group 6 length 11 (start 136)
[    0.288534] init: group 7 length 6 (start 147)
[    0.288569] init: group 8 length 4 (start 153)
[    0.288603] init: group 9 length 8 (start 157)
[    0.288637] init: group 10 length 3 (start 165)
[    0.288671] init: group 11 length 2 (start 168)
[    0.288705] init: using 4 threads to call annotated initcalls

That means the first group contains 66 initcalls which are called using 
4 threads, and, after those have finished, the second group with 33 
initcalls will be called in parallel (using the same 4 threads).

> Regards,
>
> Alexander Holler

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