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

peterz
2020-09-04T07:41:54Z

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.