Slight changes to reflect some of the changes in -current.
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20 minutes a day for the next few days and updates one or two of his favourite chapter 9 man pages as they are now WAY out of date in general.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
parent
dfb072f405
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b388e223f2
@ -43,7 +43,7 @@
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.Nm mi_switch ,
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.Nm cpu_switch ,
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.Nm cpu_throw
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.Nd switch to another process context
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.Nd switch to another thread context
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.Sh SYNOPSIS
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.In sys/param.h
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.In sys/proc.h
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@ -56,20 +56,22 @@
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.Sh DESCRIPTION
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The
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.Fn mi_switch
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function implements the machine independent prelude to a process context
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function implements the machine independent prelude to a thread context
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switch.
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It is called from only a few distinguished places in the kernel
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code as a result of the principle of non-preemtable kernel mode execution.
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The three major uses of
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The various major uses of
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.Nm
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can be enumerated as follows:
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.Bl -enum -offset indent
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.It
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from within
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.Xr sleep 9
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and
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,
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.Xr tsleep 9
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when the current process
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and
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.Xr msleep 9
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when the current thread
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voluntarily relinquishes the CPU to wait for some resource to become
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available.
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.It
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@ -79,7 +81,7 @@ when the kernel prepares a return to user-mode execution.
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This case is
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typically handled by machine dependent trap-handling code after detection
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of a change in the signal disposition of the current process, or when a
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higher priority process might be available to run.
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higher priority thread might be available to run.
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The latter event is
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communicated by the machine independent scheduling routines by calling
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the machine defined
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@ -89,11 +91,20 @@ in the signal handling code
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(see
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.Xr issignal 9 )
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if a signal is delivered that causes a process to stop.
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.It
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When a thread dies in
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.Xr thread_exit 9
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and control of the processor can be passed to the next runnable thread.
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.It
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In
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.Xr thread_suspend_check 9
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where a thread needs to stop execution due to the suspension state of
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the process as a whole.
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.El
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.Pp
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.Fn mi_switch
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records the amount of time the current process has been running in the
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process structure and checks this value against the CPU time limits
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records the amount of time the current thread has been running in the
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process structures and checks this value against the CPU time limits
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allocated to the process
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(see
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.Xr getrlimit 2 ) .
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@ -102,31 +113,41 @@ Exceeding the soft limit results in a
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signal to be posted to the process, while exceeding the hard limit will
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cause a
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.Dv SIGKILL .
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.Pp
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If the thread is still in
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.Em TDS_RUNNING
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state mi_switch will put it back onto the run queue, assuming that
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it will want to run again soon. If it is in one of the other
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states and KSE threading is enabled, the associated
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.Em KSE
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will be made available to to any higher priority threads from the same
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group, to allow them to be scheduled next.
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.Pp
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After these administrative tasks are done,
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.Fn mi_switch
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hands over control to the machine dependent routine
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.Fn cpu_switch ,
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which will perform the actual process context switch.
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which will perform the actual thread context switch.
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.Pp
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.Fn cpu_switch
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first saves the context of the current process.
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first saves the context of the current thread.
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Next, it calls
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.Fn chooseproc
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to determine which process to run next.
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Finally, it reads in the saved context of the new process and starts to
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execute the new process.
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.Fn choosethread
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to determine which thread to run next.
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Finally, it reads in the saved context of the new thread and starts to
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execute the new thread.
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.Pp
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.Fn cpu_throw
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is similar to
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.Fn cpu_switch
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except that it does not save the context of the old process.
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This function is useful when the kernel does not have an old process
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except that it does not save the context of the old thread.
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This function is useful when the kernel does not have an old thread
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context to save, such as when CPUs other than the boot CPU perform their
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first task switch, or when the kernel does not care about the state of the
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old process, such as in
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.Fn cpu_exit
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when the kernel terminates the current process and switches into a new
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process.
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old thread, such as in
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.Fn thread_exit
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when the kernel terminates the current thread and switches into a new
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thread.
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.Pp
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To protect the
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.Xr runqueue 9 ,
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@ -143,6 +143,16 @@ structure.
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.Fn crshared
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returns 0 if the credential has a reference count greater than 1;
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otherwise, 1 is returned.
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.Sh USAGE NOTES
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In general in a system call,
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you should always use the
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.Vt ucred
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associated with the
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thread doing the work rather than that of the process attached to it,
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As that associated with the thread is guaranteed to be safely accessed
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under SMP, and will remain stable for the duration of the call, even
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in the face of a multithreaded application changing the process credentials
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from another thread.
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.Sh SEE ALSO
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.Xr uihold 9
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.Sh AUTHORS
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