Messages in this thread | | | Date | Fri, 4 Sep 2020 09:41:42 +0200 | From | peterz@infradea ... | Subject | Re: [PATCH v1 6/8] x86/tsc: Use seqcount_latch_t |
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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.
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