linux-kernel - Re: [PATCH 5/5] sched_ext: Make scx_has

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Message-ID: <Z_bBrENa2luRC7nT@slm.duckdns.org>
Date: Wed, 9 Apr 2025 08:51:24 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: void@...ifault.com, multics69@...il.com, linux-kernel@...r.kernel.org,
	sched-ext@...a.com
Subject: Re: [PATCH 5/5] sched_ext: Make scx_has_op a bitmap

Hello,

On Wed, Apr 09, 2025 at 09:57:59AM +0200, Andrea Righi wrote:
> On Tue, Apr 08, 2025 at 01:06:05PM -1000, Tejun Heo wrote:
> > scx_has_op is used to encode which ops are implemented by the BPF scheduler
> > into an array of static_keys. While this saves a bit of branching overhead,
> > that is unlikely to be noticeable compared to the overall cost. As the
> > global static_keys can't work with the planned hierarchical multiple
> > scheduler support, replace the static_key array with a bitmap.
> > 
> > In repeated hackbench runs before and after static_keys removal on an AMD
> > Ryzen 3900X, I couldn't tell any measurable performance difference.
> 
> At this point I'm wondering if we should just do something like:
> 
>  #define SCX_HAS_OP(op) (scx_ops.op != NULL)
> 
> Do you think the bitmap can provide some measurable benefits? For the most
> frequently used hot paths (enqueue, dispatch, select_cpu, running,
> stopping) we likely have to fetch scx_ops.op anyway, so cache-wise the
> difference should be minimal.

Performance-wise, unlikely. However, we need to be able to enable ops in
stages during init - CPU hotplug ops need to be enabled before other ops. We
can do that by setting the op pointers in stages but the code was already
structured in a way which is easy to convert to bitmap, so that was the path
of least resistance. We surely can change it.

Thanks.

-- 
tejun