On 19-Sep-00 Bruce Evans wrote:
> On Tue, 19 Sep 2000, Andrey A. Chernov wrote:
>
>> With very latest kernel I got lots of
>>
>> microuptime() went backwards (1.3624050 -> 1.998840)
>>
>> messages just before
>>
>> Mounting root from ufs:/dev/da0s1a
>
> It really does go backwards. This is caused by the giant lock preventing
> the clock interrupt task from running soon enough. The giant lock can
> also prevent the clock interrupt task from running often enough even
> after booting. E.g., "dd if=/dev/random of=/dev/null bs=large" does
> several bad things.
It's not the Giant lock that is at fault. We give up Giant during mi_switch().
Then scheduling problem is in the way that the top-level scheduler runs. We
decide to schedule another process due to the timeslice ending during the clk
interrupt thread. In the past, this was not run as a thread, so it ran, set
the AST_* constant for needing a resched and then exited. During doreti, we
notice an AST is pending and call ast(). ast() calls userret() which notices
that a resched is needed and calls mi_switch(). In the New World Order, when
the clock interrupt occurs, we set the AST_* constant for every interrupt
before returning from sched_ithd(). This results in the actual interrupt
threads being schedule from ast(). However, when the clk ithread finishes, it
simply calls mi_switch() to enter the next process in ithd_loop(). The
need_resched() that it sets isn't handled until the next call to userret()
either via a hardware interrupt or a syscall return. Thus, the problem isn't
due to Giant, but rather to interrupt threads. As for the micruptime()
messages on boot, they only occur here on a UP kernel. On an SMP kernel I
don't get them. Also, they always occur during mi_switch() when an interrupt
thread is finishing and going back to sleep. The first such thread to be run
to generate thet error message is the irq0: clk ithread, so the clk ithread is
running fine.
> Bruce
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