Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

Joel Fernandes
2023-03-08T15:10:24Z

> On Mar 8, 2023, at 7:01 AM, Frederic Weisbecker <frederic@kernel.org> wrote:
>
> ﻿On Wed, Mar 08, 2023 at 05:52:50AM -0800, Joel Fernandes wrote:
>> Just to add to previous reply:
>>
>> One thing to consider is that it is more of a performance improvement for
>> booting in expedited mode to fallback to normal later, than a bug
>> fix. Repeated synchronize_rcu() can easily add 100s of milliseconds and to
>> remedy that — a conversion of the call from normal API to the expedited API
>> will not help.
>
> 2 things to consider:
>
> 1) Is it this about specific calls to synchronize_rcu() that repeat a lot
> and thus create such measurable impact? If so the specific callsites should
> be considered for a conversion.
>
> 2) Is it about lots of different calls to synchronize_rcu() that gather a big
> noise? Then the solution is different.
>
> Again without proper analysis, what do we know?

Again, no one disputed that proper analysis is needed. That is obvious. I was just responding to your assumption that if boot is slow, user space will also be slow. That is not a good thing to conclude because there are many factors. Slowness at boot may be considered a bug, but slowness after boot may not be (say if the user care mores for power later).

On my side I am planning to dig deeper into our boot process, but it will take time. I hope Qiuxu can do the boot analysis on his side.

Thanks.

>
> Thanks.