On Wed, Nov 28, 2018 at 10:01:46AM +0100, Peter Zijlstra wrote: > On Wed, Nov 28, 2018 at 09:56:40AM +0100, Peter Zijlstra wrote: > > On Tue, Nov 27, 2018 at 07:45:00PM +0000, Will Deacon wrote: > > > This pair of patches improves our preempt_enable() implementation slightly > > > on arm64 by making the resulting call to preempt_schedule() conditional > > > on need_resched(), which is tracked in bit 32 of the preempt count. The > > > logic is inverted so that we can detect the preempt count going to zero > > > and need_resched being set with a single CBZ instruction. > > > > > 40: a9bf7bfd stp x29, x30, [sp, #-16]! > > > 44: 910003fd mov x29, sp > > > 48: d5384101 mrs x1, sp_el0 > > > 4c: f9400820 ldr x0, [x1, #16] > > > > We load x0 which is a u64, right? > > > > > 50: d1000400 sub x0, x0, #0x1 > > > 54: b9001020 str w0, [x1, #16] > > > > But we store w0, which is the low u32, such as to not touch the high > > word which contains the preempt bit. > > > > > 58: b4000060 cbz x0, 64 <will+0x24> > > > 5c: a8c17bfd ldp x29, x30, [sp], #16 > > > 60: d65f03c0 ret > > > 64: 94000000 bl 0 <preempt_schedule> > > > 68: a8c17bfd ldp x29, x30, [sp], #16 > > > 6c: d65f03c0 ret > > > > Why not? > > > > 58: b4000060 cbnz x0, 60 <will+0x24> > > 5c: 94000000 bl 0 <preempt_schedule> > > 60: a8c17bfd ldp x29, x30, [sp], #16 > > 64: d65f03c0 ret > > > > which seems shorter. > > > > > > So it's still early, and I haven't finished (or really even started) my > > pot 'o tea, but what about: > > > > > > ldr x0, [x1, #16] // seees the high bit set -- no preempt needed > > sub x0, x0, #1 > > > > <interrupt> > > ... > > resched_curr() > > set_tsk_need_resched(); > > set_preempt_need_resched(); > > </interrupt> // sees preempt_count != 0, does not preempt > > > > str w0, [x1, #16] // stores preempt_count == 0 > > cbnz x0, 1f // taken, we still observe the high word from before > > > > 1: ret > > > > > > Which then ends with preempt_count==0, need_resched==0 and no actual > > preemption afaict. > > > > Can you use mis-matched ll x0 / sc w0 to do this same thing and detector > > the intermediate write on the high word? > > That is, something along these here lines: > > 1: ldxr x0, [x1, #16] > sub x0, x0, #1 > stxr w1, w0, [x1, #16]
^^ This guy needs a different encoding but, to be honest, I reckon we're better off just reloading the need_resched flag in the case where the count has hit zero. I'll have a play. The assembly I posted is all generated by GCC, so I can't comment on why it didn't chose your shorter sequence :) Will
