Re: [PATCH 2/3] genirq: use runtime constant to optimize handle_arch_irq access

From: Jisheng Zhang <jszhang@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Thomas Gleixner <tglx@kernel.org>,
	ardb@kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Paul Walmsley <pjw@kernel.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>, Guo Ren <guoren@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-riscv@lists.infradead.org, linux-csky@vger.kernel.org
Subject: Re: [PATCH 2/3] genirq: use runtime constant to optimize handle_arch_irq access
Date: Mon, 23 Feb 2026 21:22:44 +0800	[thread overview]
Message-ID: <aZxUpATkNI5_PbNl@xhacker> (raw)
In-Reply-To: <aZxSEoFkXMg18qLV@J2N7QTR9R3>

On Mon, Feb 23, 2026 at 01:11:46PM +0000, Mark Rutland wrote:
> On Mon, Feb 23, 2026 at 08:41:55PM +0800, Jisheng Zhang wrote:
> > On Sun, Feb 22, 2026 at 11:06:11PM +0100, Thomas Gleixner wrote:
> > > On Fri, Feb 20 2026 at 17:09, Jisheng Zhang wrote:
> > > > Currently, on GENERIC_IRQ_MULTI_HANDLER platforms, the handle_arch_irq
> > > > is a pointer which is set during booting, and every irq processing needs
> > > > to access it, so it sits in hot code path. We can use the
> > > > runtime constant mechanism which was introduced by Linus to speed up
> > > > its accessing.
> > > 
> > > The proper solution is to use a static call and update it in
> > > set_handle_irq().  That removes the complete indirect call issue from
> > > the hot path.
> > 
> > + Ard, Mark,
> > 
> > Good idea. The remaining problem is no static call support for current
> > GENERIC_IRQ_MULTI_HANDLER (or similar, arm64 e.g) platforms.
> 
> There are various reasons for not supporting static calls, and in
> general we end up having to have a fall-back path that's *more*
> expensive than just loading the pointer.

indeed, if arch doesn't support static call, the fall-back addes one
more loading overhead.

> 
> > For arm64, Ard tried to add the static call support[1] in 2021, but
> > Mark concerned "compiler could easily violate our expectations in
> > future"[2],
> 
> To be clear, that's ONE specific concern, not the ONLY reason.
> 
> > and asked for static calls "critical rather than a nice-to-have"
> > usage.
> > 
> > Hi Ard, Mark,
> > 
> > Could this irq performance improvement be used as a "critical" usage for
> > arm64 static call? Per my test, about 6.5% improvement was seen on quad CA55.
> 
> As per my other mail, does this meaningfully affect a real workload?

This improves generic irq processcing, I think all real workload is affected.

> 
> > Another alternative: disable static call if CFI is enabled, and give
> > the platform/SoC users chance to enable static call to benefit from
> > it.
> 
> Who is this actually going to matter to?
> 