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Slight changes to reflect some of the changes in -current.

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mi_switch(9) is still wildly innacurate. I suggest that every kernel
developer takes 20 minutes a day for the next few days and updates one or
two of his favourite chapter 9 man pages as they are now WAY out of date
in general. I will add a couple of KSE related pages soon.
This commit is contained in:
Julian Elischer 2002-07-08 07:34:46 +00:00
parent dfb072f405
commit b388e223f2
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=99576
2 changed files with 51 additions and 20 deletions
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61
share/man/man9/mi_switch.9
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@ -43,7 +43,7 @@
.Nm mi_switch ,
.Nm cpu_switch ,
.Nm cpu_throw
.Nd switch to another process context
.Nd switch to another thread context
.Sh SYNOPSIS
.In sys/param.h
.In sys/proc.h
@ -56,20 +56,22 @@
.Sh DESCRIPTION
The
.Fn mi_switch
function implements the machine independent prelude to a process context
function implements the machine independent prelude to a thread context
switch.
It is called from only a few distinguished places in the kernel
code as a result of the principle of non-preemtable kernel mode execution.
The three major uses of
The various major uses of
.Nm
can be enumerated as follows:
.Bl -enum -offset indent
.It
from within
.Xr sleep 9
and
,
.Xr tsleep 9
when the current process
and
.Xr msleep 9
when the current thread
voluntarily relinquishes the CPU to wait for some resource to become
available.
.It
@ -79,7 +81,7 @@ when the kernel prepares a return to user-mode execution.
This case is
typically handled by machine dependent trap-handling code after detection
of a change in the signal disposition of the current process, or when a
higher priority process might be available to run.
higher priority thread might be available to run.
The latter event is
communicated by the machine independent scheduling routines by calling
the machine defined
@ -89,11 +91,20 @@ in the signal handling code
(see
.Xr issignal 9 )
if a signal is delivered that causes a process to stop.
.It
When a thread dies in
.Xr thread_exit 9
and control of the processor can be passed to the next runnable thread.
.It
In
.Xr thread_suspend_check 9
where a thread needs to stop execution due to the suspension state of
the process as a whole.
.El
.Pp
.Fn mi_switch
records the amount of time the current process has been running in the
process structure and checks this value against the CPU time limits
records the amount of time the current thread has been running in the
process structures and checks this value against the CPU time limits
allocated to the process
(see
.Xr getrlimit 2 ) .
@ -102,31 +113,41 @@ Exceeding the soft limit results in a
signal to be posted to the process, while exceeding the hard limit will
cause a
.Dv SIGKILL .
.Pp
If the thread is still in
.Em TDS_RUNNING
state mi_switch will put it back onto the run queue, assuming that
it will want to run again soon. If it is in one of the other
states and KSE threading is enabled, the associated
.Em KSE
will be made available to to any higher priority threads from the same
group, to allow them to be scheduled next.
.Pp
After these administrative tasks are done,
.Fn mi_switch
hands over control to the machine dependent routine
.Fn cpu_switch ,
which will perform the actual process context switch.
which will perform the actual thread context switch.
.Pp
.Fn cpu_switch
first saves the context of the current process.
first saves the context of the current thread.
Next, it calls
.Fn chooseproc
to determine which process to run next.
Finally, it reads in the saved context of the new process and starts to
execute the new process.
.Fn choosethread
to determine which thread to run next.
Finally, it reads in the saved context of the new thread and starts to
execute the new thread.
.Pp
.Fn cpu_throw
is similar to
.Fn cpu_switch
except that it does not save the context of the old process.
This function is useful when the kernel does not have an old process
except that it does not save the context of the old thread.
This function is useful when the kernel does not have an old thread
context to save, such as when CPUs other than the boot CPU perform their
first task switch, or when the kernel does not care about the state of the
old process, such as in
.Fn cpu_exit
when the kernel terminates the current process and switches into a new
process.
old thread, such as in
.Fn thread_exit
when the kernel terminates the current thread and switches into a new
thread.
.Pp
To protect the
.Xr runqueue 9 ,

10
share/man/man9/ucred.9
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@ -143,6 +143,16 @@ structure.
.Fn crshared
returns 0 if the credential has a reference count greater than 1;
otherwise, 1 is returned.
.Sh USAGE NOTES
In general in a system call,
you should always use the
.Vt ucred
associated with the
thread doing the work rather than that of the process attached to it,
As that associated with the thread is guaranteed to be safely accessed
under SMP, and will remain stable for the duration of the call, even
in the face of a multithreaded application changing the process credentials
from another thread.
.Sh SEE ALSO
.Xr uihold 9
.Sh AUTHORS