linux-kernel - Re: [RFC PATCH 85/86] treewide: drivers: remove cond

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Date:   Wed, 8 Nov 2023 00:48:57 +0000
From:   Chris Packham <Chris.Packham@...iedtelesis.co.nz>
To:     Ankur Arora <ankur.a.arora@...cle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
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Subject: Re: [RFC PATCH 85/86] treewide: drivers: remove cond_resched()


On 8/11/23 12:08, Ankur Arora wrote:
> There are broadly three sets of uses of cond_resched():
>
> 1.  Calls to cond_resched() out of the goodness of our heart,
>      otherwise known as avoiding lockup splats.
>
> 2.  Open coded variants of cond_resched_lock() which call
>      cond_resched().
>
> 3.  Retry or error handling loops, where cond_resched() is used as a
>      quick alternative to spinning in a tight-loop.
>
> When running under a full preemption model, the cond_resched() reduces
> to a NOP (not even a barrier) so removing it obviously cannot matter.
>
> But considering only voluntary preemption models (for say code that
> has been mostly tested under those), for set-1 and set-2 the
> scheduler can now preempt kernel tasks running beyond their time
> quanta anywhere they are preemptible() [1]. Which removes any need
> for these explicitly placed scheduling points.
>
> The cond_resched() calls in set-3 are a little more difficult.
> To start with, given it's NOP character under full preemption, it
> never actually saved us from a tight loop.
> With voluntary preemption, it's not a NOP, but it might as well be --
> for most workloads the scheduler does not have an interminable supply
> of runnable tasks on the runqueue.
>
> So, cond_resched() is useful to not get softlockup splats, but not
> terribly good for error handling. Ideally, these should be replaced
> with some kind of timed or event wait.
> For now we use cond_resched_stall(), which tries to schedule if
> possible, and executes a cpu_relax() if not.
>
> The cond_resched() calls here are all kinds. Those from set-1
> or set-2 are quite straight-forward to handle.
>
> There are quite a few from set-3, where as noted above, we
> use cond_resched() as if it were a amulent. Which I supppose
> it is, in that it wards off softlockup or RCU splats.
>
> Those are now cond_resched_stall(), but in most cases, given
> that the timeouts are in milliseconds, they could be easily
> timed waits.

For i2c-mpc.c:

It looks as the code in question could probably be converted to 
readb_poll_timeout(). If I find sufficient round-tuits I might look at 
that. Regardless in the context of the tree-wide change ...

Reviewed-by: Chris Packham <chris.packham@...iedtelesis.co.nz>