Re: [PATCH] sched_ext: Separate lock and first_task into distinct cache lines in scx_dispatch_q

[Tejun Heo <tj@kernel.org>]

On Sat, Feb 28, 2026 at 01:06:47PM +0000, David Carlier wrote:
> lock (write-heavy) and first_task (read-mostly, lockless RCU peek) share
> the same cache line in struct scx_dispatch_q. Every lock acquire/release
> by a dispatching CPU invalidates the line for all CPUs performing
> lockless first_task peeks, causing unnecessary cache coherence traffic,
> especially across NUMA nodes.
> 
> Add ____cacheline_aligned_in_smp to first_task to place it on its own
> cache line, eliminating this false sharing on SMP systems. On
> uniprocessor builds the annotation is a no-op, so no space is wasted.
> 
> On SMP, the trade-off is increased struct size: each scx_dispatch_q
> grows by up to ~56 bytes of padding. There are two instances embedded
> per-CPU in scx_rq (local_dsq and bypass_dsq), plus any dynamically
> allocated custom DSQs, so the total overhead scales with the number of
> CPUs and active DSQs.

But first_task is read-mostly. How could it be? David, from now on, I'm not
going to apply these patches unless you provide backing experimental data.

Thanks.

-- 
tejun