Re: [PATCH v1 6/8] x86/tsc: Use seqcount_latch_t

peterz
2020-09-04T08:03:30Z

On Fri, Sep 04, 2020 at 09:41:42AM +0200, peterz@infradead.org wrote:
> On Thu, Aug 27, 2020 at 01:40:42PM +0200, Ahmed S. Darwish wrote:
>
> > __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
> > {
> > + seqcount_latch_t *seqcount;
> > int seq, idx;
> >
> > preempt_disable_notrace();
> >
> > + seqcount = &this_cpu_ptr(&cyc2ns)->seq;
> > do {
> > - seq = this_cpu_read(cyc2ns.seq.sequence);
> > + seq = raw_read_seqcount_latch(seqcount);
> > idx = seq & 1;
> >
> > data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
> > data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
> > data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
> >
> > - } while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence)));
> > + } while (read_seqcount_latch_retry(seqcount, seq));
> > }
>
> So I worried about this change, it obviously generates worse code. But I
> was not expecting this:
>
> Before:
>
> 196: 0000000000000110 189 FUNC GLOBAL DEFAULT 1 native_sched_clock
>
> After:
>
> 195: 0000000000000110 399 FUNC GLOBAL DEFAULT 1 native_sched_clock
>
> That's _210_ bytes extra!!
>
> If you look at the disassembly of the thing after it's a complete
> trainwreck.

The below delta fixes it again.

---
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -68,21 +68,19 @@ early_param("tsc_early_khz", tsc_early_k

__always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
{
- seqcount_latch_t *seqcount;
int seq, idx;

preempt_disable_notrace();

- seqcount = &this_cpu_ptr(&cyc2ns)->seq;
do {
- seq = raw_read_seqcount_latch(seqcount);
+ seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
idx = seq & 1;

data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
data->cyc2ns_mul = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
data->cyc2ns_shift = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);

- } while (read_seqcount_latch_retry(seqcount, seq));
+ } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
}

__always_inline void cyc2ns_read_end(void)