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SubjectRe[2]: Latency measurements
Hi Robert,
Monday, October 01, 2001, 11:47:31 PM, you wrote:

>> These are the longest held locks on my system
>> (PII 233 UP, 32MB RAM, SVGA 16bit color fb console, X)
>> Kernel: 2.4.10 + ext3 + preemption
>> I am very willing to test any patches to reduce latency.
>> 418253 BKL 1 712/tty_io.c c01b41c5 714/tty_io.c
>> 222609 BKL 1 712/tty_io.c c01b41c5 697/sched.c
>> 152903 spin_lock 5 547/sched.c c0114fd5 714/tty_io.c
>> 132422 BKL 5 712/tty_io.c c01b41c5 714/tty_io.c
>> 104548 BKL 1 712/tty_io.c c01b41c5 1380/sched.c

RL> Unfortunately there isn't much we can do about any of those locks.

RL> The locks in tty_io.c are have to be held, the fact you are using a
RL> framebuffer makes it a lot worse, though. If there is an accelerated fb
RL> for your video card, I would suggest that.

That is a BKL which we are trying to get rid of.
What deadlock is prevented by lock_kernel()
in tty_io.c:712?

write() call there is actually a tty->ldisc.write().
Is it possible to move lock into tty->ldisc.write()
and make it a spinlock? I'd like to try, but I admit
I failed to track what fn ptr is placed in ldisc.write
in my case (fb console) :-(

>> 222609 BKL 1 712/tty_io.c 697/sched.c
I don't quite understand how locked region can start in
712/tty_io.c and end in 697/sched.c?

This is strange too:
>> 152903 spin_lock 5 547/sched.c 714/tty_io.c
spinlock? Unlocked by unlock_kernel()???
Best regards, VDA

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