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Re: Scheduling problem - need some help here



On 28/06/2020 16:11, Anders Magnusson wrote:
Hi,

there is a problem (on vax) that I do not really understand.  Greg Oster
filed a PR on it (#55415).

A while ago ad@ removed the  "(ci)->ci_want_resched = 1;" from
cpu_need_resched() in vax/include/cpu.h.
And as I read the code (in kern_runq.c) it shouldn't be needed,
ci_want_resched should be set already when the macro cpu_need_resched()
is invoked.

But; without setting cpu_need_resched=1 the vax performs really bad (as
described in the PR).

cpu_need_resched may have multiple values nowadays, setting it to 1 will
effectively clear out other flags, which is probably what makes it work.

Anyone know what os going on here (and can explain it to me)?

I'm no expert here, but I think the expectation is that each platform
has its own method to signal "ast pending" and eventually call userret
(and preempt) when it's set - see setsoftast/aston.

As I don't understand vax I don't know what

	197 #define cpu_signotify(l)     mtpr(AST_OK,PR_ASTLVL)

is expected to do, but somehow it should result in userret() being called.

Other points are:

- vax cpu_need_resched doesn't seem to differentiate between locally
  running lwp and an lwp running on another cpu.

- I can't see how hardclock would result in userret being called, but
  like I said - I don't know vax.


http://src.illumos.org/source/xref/netbsd-src/sys/arch/vax/vax/intvec.S#311

I believe ci_want_resched is an MI variable for the scheduler which is
why its use in vax cpu_need_resched got removed.

End of ramblings...

Nick


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