linux-kernel - Re: [PATCH v1 6/8] x86/tsc: Use seqcount_latch

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Date:   Fri, 4 Sep 2020 09:41:42 +0200
From:   peterz@...radead.org
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v1 6/8] x86/tsc: Use seqcount_latch_t

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.