linux-kernel - Re: [PATCH] ACPICA: Fix a soft-lockup on large systems

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Date:   Tue, 29 Sep 2020 17:01:14 -0400
From:   Qian Cai <cai@...hat.com>
To:     "Kaneda, Erik" <erik.kaneda@...el.com>,
        "Moore, Robert" <robert.moore@...el.com>,
        "Wysocki, Rafael J" <rafael.j.wysocki@...el.com>
Cc:     Len Brown <lenb@...nel.org>,
        "linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
        "devel@...ica.org" <devel@...ica.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH] ACPICA: Fix a soft-lockup on large systems

On Tue, 2020-09-29 at 19:55 +0000, Kaneda, Erik wrote:
> This is acpica code and cond_resched is specific to Linux so we cannot accept
> this in its current form.

Do you have any suggestion?

> 
> The execution time of acpi_ns_walk_namespace is relative to the size of the
> acpi namespace. This is determined by the size of firmware..
> If the actual culprit was the traversing the ACPI namespace, you should have a
> soft lock up on acpi_load_tables which is the function that populates the ACPI
> namespace. Your stack trace shows that Linux was able to get past this point.
> Therefore, I'm led to think that the actual problem is the combination of
> walking the namespace + the handler invoked.
> 
> What happens if you add the cond_resched in acpi_bus_check_add?

This also works fine.

--- a/drivers/acpi/scan.c
+++ b/drivers/acpi/scan.c
@@ -1881,6 +1881,7 @@ static acpi_status acpi_bus_check_add(acpi_handle handle,
u32 lvl_not_used,
                return AE_OK;
        }
 
+       cond_resched();
        acpi_add_single_object(&device, handle, type, sta);
        if (!device)
                return AE_CTRL_DEPTH;

> 
> Out of curiosity, does calling cond_resched guarantee that the acpi_init call
> will finish before other kernel components that depend on ACPI are
> initialized?

I don't really see how it could break the dependencies. cond_resched() is just
to avoid stalling the CPU.