linux-kernel - Re: [patch] tlb flush_data: replace per

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Message-ID: <20090112235156.GC10720@gambetta>
Date:	Tue, 13 Jan 2009 00:51:56 +0100
From:	Frederik Deweerdt <frederik.deweerdt@...og.eu>
To:	Mike Travis <travis@....com>
Cc:	mingo@...e.hu, andi@...stfloor.org, tglx@...utronix.de,
	hpa@...or.com, linux-kernel@...r.kernel.org
Subject: Re: [patch] tlb flush_data: replace per_cpu with an array

On Mon, Jan 12, 2009 at 02:54:25PM -0800, Mike Travis wrote:
> Frederik Deweerdt wrote:
> > Hi,
> > 
> > On x86_64 flush tlb data is stored in per_cpu variables. This is
> > unnecessary because only the first NUM_INVALIDATE_TLB_VECTORS entries
> > are accessed.
> > This patch aims at making the code less confusing (there's nothing
> > really "per_cpu") by using a plain array. It also would save some memory
> > on most distros out there (Ubuntu x86_64 has NR_CPUS=64 by default).
> > 
> > Regards,
> > Frederik
> > 
> > Signed-off-by: Frederik Deweerdt <frederik.deweerdt@...og.eu>
> 
> Here is the net change in memory usage with this patch on a allyesconfig
> with NR_CPUS=4096.
Yes, this point wrt. memory was based on my flawed understanding of how
per_cpu actually allocates the data. There is however 1) a confusing use
of per_cpu removed, 2) faster access to the flush data.

> 
> ====== Data (-l 500)
> 
>     1 - initial
>     2 - change-flush-tlb
> 
>   .1.    .2.  .delta.
>     0   5120   +5120      .  flush_state(.bss)
> 
> ====== Sections (-l 500)
> 
>         .1.    .2.  .delta.
>    12685496 12693688       +8192 +0.06%  .bss
>     1910176  1909408        -768 -0.04%  .data.percpu
> 
I get :
Initial
 size ./arch/x86/kernel/tlb_64.o
   text	   data	    bss	    dec	    hex	filename
   1667	    136	      8	   1811	    713	./arch/x86/kernel/tlb_64.o

After
 size ./arch/x86/kernel/tlb_64.o 
   text	   data	    bss	    dec	    hex	filename
   1598	      8	   1088	   2694	    a86	./arch/x86/kernel/tlb_64.o

    -69    -128   +1080    +883

But I'm not sure those numbers are that relevant, as the percpu part
will be allocated at runtime.

Regards,
Frederik

> > 
> > diff --git a/arch/x86/kernel/tlb_64.c b/arch/x86/kernel/tlb_64.c
> > index f8be6f1..c177a1f 100644
> > --- a/arch/x86/kernel/tlb_64.c
> > +++ b/arch/x86/kernel/tlb_64.c
> > @@ -33,7 +33,7 @@
> >   *	To avoid global state use 8 different call vectors.
> >   *	Each CPU uses a specific vector to trigger flushes on other
> >   *	CPUs. Depending on the received vector the target CPUs look into
> > - *	the right per cpu variable for the flush data.
> > + *	the right array slot for the flush data.
> >   *
> >   *	With more than 8 CPUs they are hashed to the 8 available
> >   *	vectors. The limited global vector space forces us to this right now.
> > @@ -54,7 +54,7 @@ union smp_flush_state {
> >  /* State is put into the per CPU data section, but padded
> >     to a full cache line because other CPUs can access it and we don't
> >     want false sharing in the per cpu data segment. */
> > -static DEFINE_PER_CPU(union smp_flush_state, flush_state);
> > +static union smp_flush_state flush_state[NUM_INVALIDATE_TLB_VECTORS];
> >  
> >  /*
> >   * We cannot call mmdrop() because we are in interrupt context,
> > @@ -129,7 +129,7 @@ asmlinkage void smp_invalidate_interrupt(struct pt_regs *regs)
> >  	 * Use that to determine where the sender put the data.
> >  	 */
> >  	sender = ~regs->orig_ax - INVALIDATE_TLB_VECTOR_START;
> > -	f = &per_cpu(flush_state, sender);
> > +	f = &flush_state[sender];
> >  
> >  	if (!cpu_isset(cpu, f->flush_cpumask))
> >  		goto out;
> > @@ -169,7 +169,7 @@ void native_flush_tlb_others(const cpumask_t *cpumaskp, struct mm_struct *mm,
> >  
> >  	/* Caller has disabled preemption */
> >  	sender = smp_processor_id() % NUM_INVALIDATE_TLB_VECTORS;
> > -	f = &per_cpu(flush_state, sender);
> > +	f = &flush_state[sender];
> >  
> >  	/*
> >  	 * Could avoid this lock when
> > @@ -205,8 +205,8 @@ static int __cpuinit init_smp_flush(void)
> >  {
> >  	int i;
> >  
> > -	for_each_possible_cpu(i)
> > -		spin_lock_init(&per_cpu(flush_state, i).tlbstate_lock);
> > +	for (i = 0; i < ARRAY_SIZE(flush_state); i++)
> > +		spin_lock_init(&flush_state[i].tlbstate_lock);
> >  
> >  	return 0;
> >  }
> > --
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