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: <Z-6XkEpiXg4stqLY@gmail.com>
Date: Thu, 3 Apr 2025 16:13:36 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Nikolay Borisov <nik.borisov@...e.com>
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()


* Nikolay Borisov <nik.borisov@...e.com> wrote:

> 
> 
> On 3.04.25 г. 16:38 ч., Ingo Molnar wrote:
> > 
> > * Nikolay Borisov <nik.borisov@...e.com> wrote:
> > 
> > > 
> > > 
> > > On 28.03.25 г. 15:26 ч., Ingo Molnar wrote:
> > > > Simplifies the code and improves code generation a bit:
> > > > 
> > > >      text	   data	    bss	    dec	    hex	filename
> > > >     14769	   1017	   4112	  19898	   4dba	alternative.o.before
> > > >     14742	   1017	   4112	  19871	   4d9f	alternative.o.after
> > > > 
> > > > Signed-off-by: Ingo Molnar <mingo@...nel.org>
> > > > ---
> > > >    arch/x86/kernel/alternative.c | 11 +++++------
> > > >    1 file changed, 5 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> > > > index 1df8fac6740d..5293929488f0 100644
> > > > --- a/arch/x86/kernel/alternative.c
> > > > +++ b/arch/x86/kernel/alternative.c
> > > > @@ -2750,6 +2750,9 @@ static void smp_text_poke_batch_process(void)
> > > >    		if (unlikely(!atomic_dec_and_test(refs)))
> > > >    			atomic_cond_read_acquire(refs, !VAL);
> > > >    	}
> > > > +
> > > > +	/* They are all completed: */
> > > > +	text_poke_array.nr_entries = 0;
> > > >    }
> > > >    static void __smp_text_poke_batch_add(void *addr, const void *opcode, size_t len, const void *emulate)
> > > > @@ -2857,20 +2860,16 @@ static bool text_poke_addr_ordered(void *addr)
> > > >    void smp_text_poke_batch_finish(void)
> > > >    {
> > > > -	if (text_poke_array.nr_entries) {
> > > > +	if (text_poke_array.nr_entries)
> > > >    		smp_text_poke_batch_process();
> > > > -		text_poke_array.nr_entries = 0;
> > > > -	}
> > > >    }
> > > 
> > > This function becomes trivial, why not simply move the check into
> > > smp_text_poke_batch_process and rename it to smp_text_poke_batch_finish ?
> > 
> > Yeah, that's pretty much what happens in patch #47:
> > 
> >    x86/alternatives: Remove 'smp_text_poke_batch_flush()'
> 
> Well, that patch removes poke_batch_flush but still retains
> poke_batch_finish + poke_batch_process. My suggestion is to eliminate
> poke_batch_process name and turn it into poke_batch_finish by moving the
> check from poke_batch_finish into poke_batch_process.

So, in the context of the full tree at:

  git://git.kernel.org/pub/scm/linux/kernel/git/mingo/tip.git WIP.x86/alternatives

Standalone smp_text_poke_batch_process() is still needed, because 
smp_text_poke_batch_add() uses it to drive the whole 'batching' 
machinery.

If smp_text_poke_batch_process() finishes for each call, if I 
understand your suggestion correctly, that reduces the amount of 
batching done, which is a disadvantage.

Note how right now it's possible to do something like this:

	smp_text_poke_batch_add(1);
	smp_text_poke_batch_add(1);
	smp_text_poke_batch_add(1);
	smp_text_poke_batch_add(1);
	smp_text_poke_batch_finish();

This results in a single call to smp_text_poke_batch_process(), with 4 
entries, a single 4-entry flush in essence.

Thanks,

	Ingo