Date: Thu, 27 Apr 2000 01:47:41 -0400 (EDT) From: Luoqi Chen <luoqi@watermarkgroup.com> To: green@FreeBSD.ORG, hackers@FreeBSD.ORG Subject: Re: lock-ups due to the scheduler Message-ID: <200004270547.e3R5lfi24004@lor.watermarkgroup.com>
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This is quite interesting. I'm no scheduler expert, but my understanding
is priority < PUSER won't degrade and is only set in kernel mode after
waking up from a sleep. In user mode, processes should always have priority
p_usrpri >= PUSER, it is obviously not true for a negative nice value:
> newpriority = PUSER + p->p_estcpu / INVERSE_ESTCPU_WEIGHT +
> NICE_WEIGHT * p->p_nice;
We should probably offset p->p_nice by PRIO_MIN,
> newpriority = PUSER + p->p_estcpu / INVERSE_ESTCPU_WEIGHT +
> NICE_WEIGHT * (p->p_nice - PRIO_MIN);
To fully utilize the 20 out of 32 run queues for user priorities, we might
want to change NICE_WEIGHT from 2 to 1, and upper limit of p_estcpu to
#define ESTCPULIM(e) \
min((e),
INVERSE_ESTCPU_WEIGHT * (NICE_WEIGHT * (PRIO_MAX - PRIO_MIN) - PPQ) + \
INVERSE_ESTCPU_WEIGHT - 1)
so that a cpu hog at nice 0 would have about the same priority as a low
cpu usage nice +20 process.
-lq
> I dropped hints that there may be issues about 3 weeks ago, as my machine
> had locked up for apparently no reason, and I had no idea why until
> recently. It seems that it has everything to do with running things that
> use lots of CPU at a very high priority (I use -20).
>
> I've been struggling for a few days with this... and what happens is that
> the kernel never executes any user processes (or does so so very rarely
> that I really can't detect it at all, in any application, the execution).
>
> I'm not really sure what's happening, but it definitely is with the
> scheduler: if I cap the scheduler's priority computation on the lower
> end to keep user processes from not executing with a p_priority < PUSER,
> the system can get slightly unresponsive, but it does not lock up.
> The modifications I made to allow prevention of this follow my signature.
>
> This is a deadlock-type situation, and I can reproduce it at will, so I'll
> try to explain the steps I can reproduce it with.
>
> 1. start XMMS at -20 priority, and play something
> 2. XMMS is decoding audio and other random things, nothing huge, but at
> about 15-20% CPU. XMMS sends decoded mp3 (archives of CDs I own) to
> the EsounD daemon (esd), which takes 2-3% cpu or so, and XFree86
> itself takes a good 5-10% cpu.
> 3. Start a "Visualization" plugin, which basically takes XMMS to full
> CPU usage (as much as it can get), and things lock up. XMMS is the
> curproc for every single time I've polled it (using DDB, for example),
> and I stop hearing audio. XFree86 would be doing the X11 servering,
> and esd would mostly be writing to the audio device or reading from its
> socket, so usually in PRIBIO or PSOCK.
>
> At this point, the system is really locked up, and there's nothing I can
> do. I can, however, get a coredump and have the entire system state at
> this point. I'm certain that that other people here will be able to try
> the same tests, of course on 5.0-CURRENT, and possibly reproduce them
> exactly the same as it happens for me. I can grovel in a coredump to
> get information about the system as it was running at the time, so if
> anyone can provide hints as to where to check for what happened that
> makes things lock up nowadays, I'll be grateful, and I'll be able to
> try almost anything to get this fixed.
>
> If you're familiar with the scheduler area of the system, please help.
> I have noone's arm to twist or anything of the sort, so I'm really
> going out on a limb hoping someone will be able to try to help me fix
> this. Note that I've taken my HZ=1000 line out of my kernel config,
> so I'm running at a standard hz = 100 and a kern.quantum of 20000.
>
> --
> Brian Fundakowski Feldman \ FreeBSD: The Power to Serve! /
> green@FreeBSD.org `------------------------------'
>
> Index: kern_synch.c
> ===================================================================
> RCS file: /usr2/ncvs/src/sys/kern/kern_synch.c,v
> retrieving revision 1.89
> diff -u -u -r1.89 kern_synch.c
> --- kern_synch.c 2000/03/28 18:06:42 1.89
> +++ kern_synch.c 2000/04/27 00:55:21
> @@ -903,6 +903,10 @@
> maybe_resched(p);
> }
>
> +static int priority_lower_cap = 0;
> +SYSCTL_INT(_debug, OID_AUTO, enable_priority_lower_cap, CTLFLAG_RW,
> + &priority_lower_cap, 0, "");
> +
> /*
> * Compute the priority of a process when running in user mode.
> * Arrange to reschedule if the resulting priority is better
> @@ -917,6 +921,12 @@
> if (p->p_rtprio.type == RTP_PRIO_NORMAL) {
> newpriority = PUSER + p->p_estcpu / INVERSE_ESTCPU_WEIGHT +
> NICE_WEIGHT * p->p_nice;
> + if (priority_lower_cap && newpriority < PUSER) {
> + if (p == curproc)
> + uprintf("kernel: tried to use priority %d\n",
> + newpriority);
> + newpriority = PUSER;
> + }
> newpriority = min(newpriority, MAXPRI);
> p->p_usrpri = newpriority;
> }
>
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