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Date: Wed, 13 Mar 2024 22:49:10 +0000
From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Florian Fainelli <f.fainelli@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Anna-Maria Behnsen <anna-maria@linutronix.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	linux-kernel@vger.kernel.org, kernel-team@meta.com,
	paulmck@kernel.org, mingo@kernel.org, tglx@linutronix.de,
	rcu@vger.kernel.org, neeraj.upadhyay@amd.com, urezki@gmail.com,
	qiang.zhang1211@gmail.com, frederic@kernel.org,
	bigeasy@linutronix.de, chenzhongjin@huawei.com,
	yangjihong1@huawei.com, rostedt@goodmis.org,
	Justin Chen <justin.chen@broadcom.com>
Subject: Re: Unexplained long boot delays [Was Re: [GIT PULL] RCU changes for
 v6.9]
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Sender: Russell King (Oracle) <linux@armlinux.org.uk>

On Wed, Mar 13, 2024 at 03:04:26PM -0700, Florian Fainelli wrote:
> On 3/13/24 14:59, Russell King (Oracle) wrote:
> > On Wed, Mar 13, 2024 at 02:30:43PM -0700, Florian Fainelli wrote:
> > > I will try to provide multiple answers for the sake of everyone having the
> > > same context. Responding to Linus' specifically and his suggestion to use
> > > "initcall_debug", this is what it gave me:
> > > 
> > > [    6.970669] ata1: SATA link down (SStatus 0 SControl 300)
> > > [  166.136366] probe of unimac-mdio-0:01 returned 0 after 159216218 usecs
> > > [  166.142931] unimac-mdio unimac-mdio.0: Broadcom UniMAC MDIO bus
> > > [  166.148900] probe of unimac-mdio.0 returned 0 after 159243553 usecs
> > > [  166.155820] probe of f0480000.ethernet returned 0 after 159258794 usecs
> > > [  166.166427] ehci-brcm f0b00300.ehci_v2: EHCI Host Controller
> > > 
> > > Also got another occurrence happening resuming from suspend to DRAM with:
> > > 
> > > [   22.570667] brcmstb-dpfe 9932000.dpfe-cpu: PM: calling
> > > platform_pm_resume+0x0/0x54 @ 1574, parent: rdb
> > > [  181.643809] brcmstb-dpfe 9932000.dpfe-cpu: PM:
> > > platform_pm_resume+0x0/0x54 returned 0 after 159073134 usecs
> > > 
> > > and also with the PCIe root complex driver:
> > > 
> > > [   18.266279] brcm-pcie f0460000.pcie: PM: calling
> > > brcm_pcie_resume_noirq+0x0/0x164 @ 1597, parent: platform
> > > [  177.457219] brcm-pcie f0460000.pcie: clkreq-mode set to default
> > > [  177.457225] brcm-pcie f0460000.pcie: link up, 2.5 GT/s PCIe x1 (!SSC)
> > > [  177.457231] brcm-pcie f0460000.pcie: PM: brcm_pcie_resume_noirq+0x0/0x164
> > > returned 0 after 159190939 usecs
> > > [  177.457257] pcieport 0000:00:00.0: PM: calling
> > > pci_pm_resume_noirq+0x0/0x160 @ 33, parent: pci0000:00
> > > 
> > > Surprisingly those drivers are consistently reproducing the failures I am
> > > seeing so at least this gave me a clue as to where the problem is.
> > > 
> > > There were no changes to drivers/net/ethernet/broadcom/genet/, the two
> > > changes done to drivers/net/mdio/mdio-bcm-unimac.c are correct, especially
> > > the read_poll_timeout() conversion is correct, we properly break out of the
> > > loop. The initial delay looked like a good culprit for a little while, but
> > > it is not used on the affected platforms because instead we provide a
> > > callback and we have an interrupt to signal the completion of a MDIO
> > > operation, therefore unimac_mdio_poll() is not used at all. Finally
> > > drivers/memory/brcmstb_dpfe.c also received a single change which is not
> > > functional here (.remove function change do return void).
> > > 
> > > I went back to a manual bisection and this time I believe that I have a more
> > > plausible candidate with:
> > > 
> > > 7ee988770326fca440472200c3eb58935fe712f6 ("timers: Implement the
> > > hierarchical pull model")
> > 
> > I haven't understood the code there yet, and how it would interact with
> > arch code, but one thing that immediately jumps out to me is this:
> > 
> > "    As long as a CPU is busy it expires both local and global timers. When a
> >      CPU goes idle it arms for the first expiring local timer."
> > 
> > So are local timers "armed" when they are enqueued while the cpu is
> > "busy" during initialisation, and will they expire, and will that
> > expiry be delivered in a timely manner?
> > 
> > If not, this commit is basically broken, and would be the cause of the
> > issue you are seeing. For the mdio case, we're talking about 2ms
> > polling. For the dpfe case, it looks like we're talking about 1ms
> > sleeps. I'm guessing that these end up being local timers.
> > 
> > Looking at pcie-brcmstb, there's a 100ms msleep(), and then a polling
> > for link up every 5ms - if the link was down and we msleep(5) I wonder
> > if that's triggering the same issue.
> > 
> > Why that would manifest itself on 32-bit but not 64-bit Arm, I can't
> > say. I would imagine that the same hardware timer driver is being used
> > (may be worth checking DT.) The same should be true for the interrupt
> > driver as well. There's been no changes in that code.
> 
> I just had it happen with ARM64 I was plagued by:
> 
> https://lore.kernel.org/lkml/87wmqrjg8n.fsf@somnus/T/
> 
> and my earlier bisections somehow did not have ARM64 fail, so I thought it
> was immune but it fails with about the same failure rate as ARM 32-bit.

Okay, so if that's two architectures failing, it's probably not an
architecture specific issue. It could still be a timer/interrupt driver
issue though. It would be nice to have a reproducer on x86 which would
confirm it.

Do you know the frequency that the failure happens?

-- 
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