Re: [linux-pm] Resume from Memory very slow on 2.6.37+

Rafael J. Wysocki
2011-01-19T21:29:17Z

On Wednesday, January 19, 2011, Jeff Chua wrote:
> On Wed, Jan 19, 2011 at 9:38 AM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
> > On Wednesday, January 19, 2011, Jeff Chua wrote:
> >> On Wed, Jan 19, 2011 at 4:20 AM, Rafael J. Wysocki <rjw@sisk.pl> wrote:
> >> > Please apply the appended patch (without the previous one) and post a dmesg
> >> > log containing a suspend-resume cycle with the delay. I wonder where exactly
> >> > the delay occurs.
> >> > + pr_info("%s: synchronize_rcu()\n", __func__);
> >> > synchronize_rcu();
> >> > + pr_info("%s: iounmap(%p), physaddr: %llx, size: %u\n", __func__,
> >> > + map->virt, map->phys, map->size);
> >>
> >> Rafael,
> >>
> >> The 30 seconds is due to this ...
> >>
> >> PM: Syncing filesystems ... done.
> >> Freezing user space processes ... _memory: synchronize_rcu()
> >> acpi_os_unmap_memory: iounmap(ffffc90001c22000), physaddr: bb398000, size: 4096
> >> acpi_os_unmap_memory: synchronize_rcu()
> >> acpi_os_unmap_memory: iounmap(ffffc90001c26000), physaddr: bb399000, size: 4096
> 00), physaddr: bb79e000, size: 4096
> >> Restarting tasks ... done.
> >>
> >> After this, I see my X windows.
> >
> > Are you able to say whether the synchronize_rcu() or the iounmap() calls block?
>
> It's "synchronize_rcu()" that's causing the slow down. Removing it
> solves the problem.

That confirms my theory, thanks.

> Does synchronize_rcu() triggers the limiting update rate as described
> in Documentation/RCU/whatisRCU.txt?
>
> In addition, the synchronize_rcu() API has the nice property
> of automatically limiting update rate should grace periods
> be delayed. This property results in system resilience in face
> of denial-of-service attacks.

Well, I'm not sure. Anyway, it apparently is better to avoid calling it so
many times in a row and, quite frankly, unnecessarily.

OK, I'd like you to test the two patches I'll send in replies to this
message.

[1/2] - It will go back to using ioremap_cache() for NVS pages allocations.
[2/2] - This one will move suspend_nvs_free() before device drivers' resume
callbacks (should avoid conflicts with the drivers' mappings).

Thanks,
Rafael