linux-kernel - Re: [PATCH 37/49] x86/alternatives: Move text_poke_array completion from smp_text_poke_batch_finish() and smp_text_poke_batch_flush() to smp_text_poke_batch

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Message-ID: <b7c3882f-9691-47fc-9e50-267fef9cfc06@suse.com>
Date: Thu, 3 Apr 2025 18:34:37 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Juergen Gross <jgross@...e.com>,
 "H . Peter Anvin" <hpa@...or.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, Borislav Petkov <bp@...en8.de>,
 Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 37/49] x86/alternatives: Move text_poke_array completion
 from smp_text_poke_batch_finish() and smp_text_poke_batch_flush() to
 smp_text_poke_batch_process()

On 3.04.25 г. 18:29 ч., Ingo Molnar wrote:
> 
> * Nikolay Borisov <nik.borisov@...e.com> wrote:
> 
>> I meant doing this:
>>
>> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
>> index 5b1a6252a4b9..b6a781b9de26 100644
>> --- a/arch/x86/kernel/alternative.c
>> +++ b/arch/x86/kernel/alternative.c
>> @@ -2587,12 +2587,16 @@ noinstr int smp_text_poke_int3_trap_handler(struct
>> pt_regs *regs)
>>    *               replacing opcode
>>    *     - SMP sync all CPUs
>>    */
>> -static void smp_text_poke_batch_process(void)
>> +void smp_text_poke_batch_finish(void)
>>   {
>>          unsigned char int3 = INT3_INSN_OPCODE;
>>          unsigned int i;
>>          int do_sync;
>>
>> +
>> +       if (!text_poke_array.nr_entries)
>> +               return;
> 
>> -               smp_text_poke_batch_process();
>> +               smp_text_poke_batch_finish();
> 
> I suppose we could do this - it adds one more check to
> smp_text_poke_batch_add() though.

poke_batch_finish you meant?

> 
>> AFAICS this doesn't change the semantics. I.e smp_text_poke_batch_add
>> will call poke_batch_finish iff the address to be added violates the
>> sorted order of text_poke_array. The net effect is we have 1 less
>> function name to care about.
> 
> Yeah, it doesn't change semantics, but it's a very small
> deoptimization.
 > > Mind sending a patch? It does simplify the facility some more and that
> single branch will wash away against costs like the CR3 flushes done

Given that poke_batch_finish does a cond_resched and sync_each_cpu which 
is an IPI can it even be considered a performance critical path ?

> ...
> 
> Thanks,
> 
> 	Ingo