Date: Fri, 14 Nov 2014 23:10:45 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com>, John Baldwin <jhb@FreeBSD.org> Cc: "src-committers@freebsd.org" <src-committers@FreeBSD.org>, Jung-uk Kim <jkim@FreeBSD.org>, "freebsd-arch@freebsd.org" <freebsd-arch@FreeBSD.org> Subject: suspending threads before devices [Was: svn commit: r233249 - head/sys/amd64/acpica] Message-ID: <54666FD5.6080705@FreeBSD.org> In-Reply-To: <20120322141436.GC2358@deviant.kiev.zoral.com.ua> References: <201203202037.q2KKbNfK037014@svn.freebsd.org> <201203211502.14353.jkim@FreeBSD.org> <4F6AF1CB.80902@FreeBSD.org> <201203220748.49635.jhb@freebsd.org> <20120322141436.GC2358@deviant.kiev.zoral.com.ua>
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On 22/03/2012 16:14, Konstantin Belousov wrote: > I already noted this to Jung-uk, I think that current suspend handling > is (somewhat) wrong. We shall not stop other CPUs for suspension when > they are executing some random kernel code. Rather, CPUs should be safely > stopped at the kernel->user boundary, or at sleep point, or at designated > suspend point like idle loop. > > We already are engaged into somewhat doubtful actions like restoring of %cr2, > since we might, for instance, preemt page fault handler with suspend IPI. I recently revisited this issue in the context of some suspend+resume problems that I am having with radeonkms driver. What surprised me is that the driver's suspend code has no synchronization whatsoever with its other code paths. So, I looked first at the Linux code and then at the illumos code to see how suspend is implemented there. As far as I can see, those kernels do exactly what you suggest that we do. Before suspending devices they first suspend all threads except for one that initiates the suspend. For userland threads a signal-like mechanism is used to put them in a state similar to SIGSTOP-ed one. With the kernel threads mechanisms are different between the kernels. Also, illumos freezes kernel threads after suspending the devices, not before. I think that we could start with only the userland threads initially. Do you think the SIGSTOP-like approach would be hard to implement for us? References: http://src.illumos.org/source/xref/illumos-gate/usr/src/uts/common/cpr/cpr_main.c#425 http://src.illumos.org/source/xref/illumos-gate/usr/src/uts/common/cpr/cpr_uthread.c#80 http://lxr.free-electrons.com/source/kernel/power/suspend.c#L388 http://lxr.free-electrons.com/source/kernel/power/suspend.c#L207 http://lxr.free-electrons.com/source/kernel/power/power.h#L235 http://lxr.free-electrons.com/source/kernel/power/process.c#L118 http://lxr.free-electrons.com/source/kernel/power/process.c#L27 http://lxr.free-electrons.com/source/kernel/freezer.c#L115 -- Andriy Gapon
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