linux-kernel - Re: [PATCH v4 04/12] riscv: Only send remote fences when some other CPU is online

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Message-ID: <ZZV3ZDa7eS+Ecc4i@xhacker>
Date: Wed, 3 Jan 2024 23:04:04 +0800
From: Jisheng Zhang <jszhang@...nel.org>
To: Samuel Holland <samuel.holland@...ive.com>
Cc: Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Alexandre Ghiti <alexghiti@...osinc.com>
Subject: Re: [PATCH v4 04/12] riscv: Only send remote fences when some other
 CPU is online

On Wed, Jan 03, 2024 at 10:58:01PM +0800, Jisheng Zhang wrote:
> On Tue, Jan 02, 2024 at 02:00:41PM -0800, Samuel Holland wrote:
> > If no other CPU is online, a local cache or TLB flush is sufficient.
> > These checks can be constant-folded when SMP is disabled.
> > 
> > Signed-off-by: Samuel Holland <samuel.holland@...ive.com>
> > ---
> > 
> > Changes in v4:
> >  - New patch for v4
> > 
> >  arch/riscv/mm/cacheflush.c | 4 +++-
> >  arch/riscv/mm/tlbflush.c   | 4 +++-
> >  2 files changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/riscv/mm/cacheflush.c b/arch/riscv/mm/cacheflush.c
> > index 47c485bc7df0..f7933ae88a55 100644
> > --- a/arch/riscv/mm/cacheflush.c
> > +++ b/arch/riscv/mm/cacheflush.c
> > @@ -21,7 +21,9 @@ void flush_icache_all(void)
> >  {
> >  	local_flush_icache_all();
> >  
> > -	if (riscv_use_sbi_for_rfence())
> > +	if (num_online_cpus() < 2)
> 
> with patch5, I think it's better to short cut for !SMP, I.E
> if (!IS_ENABLED(CONFIG_SMP) || num_online_cpus()) < 2)

aha, plz ignore this comment, I see the num_online_cpus() is defined as 1U for
UP.

> 
> so that the UP case can avoid a atomic read and check
> 
> > +		return;
> > +	else if (riscv_use_sbi_for_rfence())
> >  		sbi_remote_fence_i(NULL);
> >  	else
> >  		on_each_cpu(ipi_remote_fence_i, NULL, 1);
> > diff --git a/arch/riscv/mm/tlbflush.c b/arch/riscv/mm/tlbflush.c
> > index 2f18fe6fc4f3..37b3c93e3c30 100644
> > --- a/arch/riscv/mm/tlbflush.c
> > +++ b/arch/riscv/mm/tlbflush.c
> > @@ -73,7 +73,9 @@ static void __ipi_flush_tlb_all(void *info)
> >  
> >  void flush_tlb_all(void)
> >  {
> > -	if (riscv_use_sbi_for_rfence())
> > +	if (num_online_cpus() < 2)
> 
> ditto
> 
> > +		local_flush_tlb_all();
> > +	else if (riscv_use_sbi_for_rfence())
> >  		sbi_remote_sfence_vma_asid(NULL, 0, FLUSH_TLB_MAX_SIZE, FLUSH_TLB_NO_ASID);
> >  	else
> >  		on_each_cpu(__ipi_flush_tlb_all, NULL, 1);
> > -- 
> > 2.42.0
> > 
> > 
> > _______________________________________________
> > linux-riscv mailing list
> > linux-riscv@...ts.infradead.org
> > http://lists.infradead.org/mailman/listinfo/linux-riscv

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