Re: Excessive TLB flush ranges

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Christoph Hellwig <hch@lst.de>,
	Uladzislau Rezki <urezki@gmail.com>,
	Lorenzo Stoakes <lstoakes@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Baoquan He <bhe@redhat.com>, John Ogness <jogness@linutronix.de>,
	linux-arm-kernel@lists.infradead.org,
	Mark Rutland <mark.rutland@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	x86@kernel.org
Subject: Re: Excessive TLB flush ranges
Date: Tue, 16 May 2023 09:21:21 +0100	[thread overview]
Message-ID: <ZGM9AdL6nz/t9Vy6@shell.armlinux.org.uk> (raw)
In-Reply-To: <87ilcs8zab.ffs@tglx>

On Tue, May 16, 2023 at 10:18:36AM +0200, Thomas Gleixner wrote:
> On Tue, May 16 2023 at 08:37, Thomas Gleixner wrote:
> > On Mon, May 15 2023 at 22:31, Russell King wrote:
> >>> +	list_for_each_entry(va, list, list) {
> >>> +		/* flush range by one by one 'invlpg' */
> >>> +		for (addr = va->va_start; addr < va->va_end; addr += PAGE_SIZE)
> >>> +			flush_tlb_one_kernel(addr);
> >>
> >> Isn't this just the same as:
> >> 	flush_tlb_kernel_range(va->va_start, va->va_end);
> >
> > Indeed.
> 
> Actually not. At least not on x86 where it'd end up with 3 IPIs for that
> case again, instead of having one which walks the list on each CPU.

So what the best thing to do here is up to the arch implementation.
Maybe generic code shouldn't be making the decision here about
whether to call flush_tlb_one_kernel() or flush_tlb_kernel_range().

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