Re: [PATCH, RFC] protect call to set_tsk_need_resched() by the rq-lock

[Michael Buesch <mbuesch@freenet.de>]

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Quoting Ingo Molnar <mingo@elte.hu>:
> 
> * Michael Buesch <mbuesch@freenet.de> wrote:
> 
> > > > The two attached patches (one against vanilla kernel and one
> > > > against ck patchset) moves the rq-lock a few lines up in
> > > > scheduler_tick() to also protect set_tsk_need_resched().
> > > > 
> > > > Is that neccessary?
> > > 
> > > scheduler_tick() is a special case, 'current' is pinned and cannot
> > > go away, nor can it get off the runqueue.
> >
> > Can you explain in short, why this is the case, please? I don't really
> > get behind it. How are the two things enforced?
> 
> 'current' is the currently executing task and as such it wont get moved
> off the runqueue. The only way to leave the runqueue is to execute
> schedule() [or to be preempted].

Ok, I understand that.

Someone else said to me:
[quote]
"another runqueue might want to touch your runqueue
while you're in scheduler_tick" ...
"that is far more likely to hit with many many cpus and I'd
be surprised if you're the first person to get a race there"
[/quote]

What about this? Is this possible? Or did she/he/it miss a point?


It's this scenario here:
I frequently get oopses in cpu_idle(). In the two hours before
I made the patch, the machine hung twice. Since I'm running
a patched scheduler, It did not hang again. I gave em about
15 hours testing.

But maybe that's all pure luck.

>  Ingo


Unable to handle kernel paging request at virtual address 00099108
 printing eip:
b01010c0
*pde = 00000000
Oops: 0000 [#1]
SMP 
Modules linked in: nfs lockd sunrpc nvidia ipv6 ohci_hcd tuner tvaudio msp3400 bttv video_buf firmware_class btcx_risc ehci_hcd uhci_hcd usbcore intel_agp agpgart evdev
CPU:    0
EIP:    0060:[<b01010c0>]    Tainted: P      VLI
EFLAGS: 00010286   (2.6.10-rc2-ck2-nozeroram-findvmastat) 
EIP is at cpu_idle+0x31/0x3f
eax: 00000001   ebx: 00099100   ecx: 00000000   edx: 0000001d
esi: 00000000   edi: b03dff9c   ebp: b03dffe4   esp: b03dffe0
ds: 007b   es: 007b   ss: 0068
Process swapper (pid: 0, threadinfo=b03de000 task=b034db40)
Stack: 00020800 b03dfff8 b03e0898 000000bd b03e0340 b040cb80 0044f007 b0100211 
Call Trace:
 [<b0103c00>] show_stack+0x7a/0x90
 [<b0103d81>] show_registers+0x152/0x1ca
 [<b0103f86>] die+0xf4/0x178
 [<b0114556>] do_page_fault+0x42a/0x645
 [<b01038a7>] error_code+0x2b/0x30
 [<b03e0898>] start_kernel+0x13a/0x151
 [<b0100211>] 0xb0100211
Code: e0 ff ff 21 e3 eb 24 8b 0d 84 c6 40 b0 b8 26 10 10 b0 8b 15 c0 eb 34 b0 85 c9 0f 44 c8 8b 43 10 c1 e0 07 89 90 84 52 41 b0 ff d1 <8b> 43 08 a8 08 74 d5 e8 d8 7f 1f 00 eb f2 55 89 e5 56 53 fb ba 
 <0>Kernel panic - not syncing: Attempted to kill the idle task!


b010108f <cpu_idle>:
b010108f: 55                    push   %ebp
b0101090: 89 e5                 mov    %esp,%ebp
b0101092: 53                    push   %ebx
b0101093: bb 00 e0 ff ff        mov    $0xffffe000,%ebx
b0101098: 21 e3                 and    %esp,%ebx
b010109a: eb 24                 jmp    b01010c0 <cpu_idle+0x31>
b010109c: 8b 0d 84 c6 40 b0     mov    0xb040c684,%ecx
b01010a2: b8 26 10 10 b0        mov    $0xb0101026,%eax
b01010a7: 8b 15 c0 eb 34 b0     mov    0xb034ebc0,%edx
b01010ad: 85 c9                 test   %ecx,%ecx
b01010af: 0f 44 c8              cmove  %eax,%ecx
b01010b2: 8b 43 10              mov    0x10(%ebx),%eax
b01010b5: c1 e0 07              shl    $0x7,%eax
b01010b8: 89 90 84 52 41 b0     mov    %edx,0xb0415284(%eax)
b01010be: ff d1                 call   *%ecx
b01010c0: 8b 43 08              mov    0x8(%ebx),%eax
^^^^^^^^^^^^^^^^^^ OOPS. This is the check to need_resched().
b01010c3: a8 08                 test   $0x8,%al
b01010c5: 74 d5                 je     b010109c <cpu_idle+0xd>
b01010c7: e8 d8 7f 1f 00        call   b02f90a4 <schedule>
b01010cc: eb f2                 jmp    b01010c0 <cpu_idle+0x31>


Ah, and yes, the kernel is tainted. So Nvidia already received a bugreport.

-- 
Regards Michael Buesch  [ http://www.tuxsoft.de.vu ]



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