Re: [PATCHSET sched_ext/for-7.5] sched_ext: scx_qmap: Drop the compat code and fix a lowpri starvation

[Andrea Righi]

On Tue, Oct 20, 2026 at 12:00:29AM +0200, Andrea Righi wrote:
> On Mon, Oct 19, 2026 at 11:32:19AM -1000, Tejun Heo wrote:
> > Hello,
> > 
> > scx_qmap is often used to exercise sched_ext features as they land and
> > already requires the kernel it ships with, so its compat code only keeps
> > untested fallbacks around. The first patch drops it.
> > 
> > The second fixes scx_qmap's lowpri path. Nice 19 tasks coming back from
> > LOWPRI_DSQ go to SHARED_DSQ, which is consumed ahead of the FIFOs, so enough
> > nice 19 CPU hogs starve every other task until the watchdog ejects the
> > scheduler. They now go to their FIFO instead.
> > 
> > Tested with root and sub-scheduler instances under nice 0 and nice 19 CPU
> > hogs. Without the fix, the root is ejected within seconds. With it, the runs
> > complete and the FIFOs keep being dispatched.
> > 
> > Based on sched_ext/for-7.4-fixes (ff343a94e44d).
> > 
> > This patchset contains the following 2 patches.
> > 
> >  0001 sched_ext: scx_qmap: Drop the compat code
> >  0002 sched_ext: scx_qmap: Queue re-enqueued lowpri tasks on their FIFO
> 
> Both look good to me.
> 
> Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Oh... and as I sent this, I just noticed a small nit, there's still one
compat in scx_qmap.c:

    ppt = __COMPAT_ENUM_OR_ZERO("scx_consts", "SCX_RESCUE_DISABLE");

We can probably drop this as well.

Thanks,
-Andrea