AR71XX RTC

[Attilio Rao]

2010/1/26 Attilio Rao <attilio at freebsd.org>:
> 2010/1/25 Neelkanth Natu <neelnatu at yahoo.com>:
>> Hi Randall,
>>
>> --- On Mon, 1/25/10, Randall Stewart <rrs at lakerest.net> wrote:
>>
>>> From: Randall Stewart <rrs at lakerest.net>
>>> Subject: Re: AR71XX RTC
>>> To: "Neelkanth Natu" <neelnatu at yahoo.com>
>>> Cc: "M. Warner Losh" <imp at bsdimp.com>, freebsd-mips at freebsd.org
>>> Date: Monday, January 25, 2010, 12:51 PM
>>> Neil:
>>>
>>> Thanks for the patch.. it does look good since the old
>>> code was tromping part of the old threads PCB which is
>>> definitely not right ;-0 .. I do have a question for you..
>>> forgive my ignorance..
>>>
>>> What exactly are we trying to switch here. It seems
>>> that cpu_switch(...) is now being called with
>>> oldtd, newtd and mtx...
>>>
>>> I see that the thread structure has a struct mutex *td_lock
>>> as
>>> its first member. But what is this supposed to be pointing
>>> to?
>>> And when we switch are we trying to take the
>>>
>>> oldtd->td_lock and place it into the newtd->td_lock
>>>
>>> Or?
>>>
>>> What... I guess I just don't have any context for whats
>>> going on..
>>>
>>
>> I am not very sure about this myself so take this with a grain of salt:
>>
>> When a thread is being switched out because it is on a sleep queue the
>> intent is that some other thread running on a different cpu should not be
>> allowed to muck with this thread's state. To make this happen the 'td_lock'
>> of 'oldtd' is switched from what it was originally (viz. the sleep queue
>> chain lock) to 'blocked_lock'. The 'blocked_lock' is special because
>> it is always locked. This all happens in sched_switch().
>>
>> cpu_switch() is passed the original value of 'oldtd->td_lock'
>> as the third argument. When the context is switched to 'newtd' we will
>> switch back the 'oldtd->td_lock' from 'blocked_lock' to its original
>> value. And this way we don't lose any wakeups that may happen while we
>> are in sched_switch().
>>
>> At least that is my very naive understanding of it. CCing Attilio to
>> shed more light on this.
>
> Hope this little explanation may give more guidance:
> http://lists.freebsd.org/pipermail/svn-src-head/2010-January/014038.html
>
> anyways, the interesting bits in, eg. i386 are:
>
> #if defined(SMP) && defined(SCHED_ULE)
> #define SETOP           xchgl
> #define BLOCK_SPIN(reg)                                                 \
>                movl            $blocked_lock,%eax ;                    \
>        100: ;                                                          \
>                lock ;                                                  \
>                cmpxchgl        %eax,TD_LOCK(reg) ;                     \
>                jne             101f ;                                  \
>                pause ;                                                 \
>                jmp             100b ;                                  \
>        101:
> #else
> #define SETOP           movl
> #define BLOCK_SPIN(reg)
> #endif
>
> The difference with SETOP is just for providing a rel memory barriers
> vs. not providing a rel memory barrier (thus be careful, if you want
> to support ULE, to follow the same logic), while BLOCK_SPIN() does:
>
> while (oldtd->td_lock == &blocked_lock) cpu_spinwait();

s/oldtd/newtd.

Attilio


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