Commit 850bf6d5 authored by Paul E. McKenney's avatar Paul E. McKenney

doc: Set down RCU's scheduling-clock-interrupt needs

This commit documents the situations in which RCU needs the
scheduling-clock interrupt to be enabled, along with the consequences
of failing to meet RCU's needs in this area.
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 8a597d63
......@@ -2080,6 +2080,8 @@ Some of the relevant points of interest are as follows:
<li> <a href="#Scheduler and RCU">Scheduler and RCU</a>.
<li> <a href="#Tracing and RCU">Tracing and RCU</a>.
<li> <a href="#Energy Efficiency">Energy Efficiency</a>.
<li> <a href="#Scheduling-Clock Interrupts and RCU">
Scheduling-Clock Interrupts and RCU</a>.
<li> <a href="#Memory Efficiency">Memory Efficiency</a>.
<li> <a href="#Performance, Scalability, Response Time, and Reliability">
Performance, Scalability, Response Time, and Reliability</a>.
......@@ -2532,6 +2534,134 @@ I learned of many of these requirements via angry phone calls:
Flaming me on the Linux-kernel mailing list was apparently not
sufficient to fully vent their ire at RCU's energy-efficiency bugs!
<h3><a name="Scheduling-Clock Interrupts and RCU">
Scheduling-Clock Interrupts and RCU</a></h3>
<p>
The kernel transitions between in-kernel non-idle execution, userspace
execution, and the idle loop.
Depending on kernel configuration, RCU handles these states differently:
<table border=3>
<tr><th><tt>HZ</tt> Kconfig</th>
<th>In-Kernel</th>
<th>Usermode</th>
<th>Idle</th></tr>
<tr><th align="left"><tt>HZ_PERIODIC</tt></th>
<td>Can rely on scheduling-clock interrupt.</td>
<td>Can rely on scheduling-clock interrupt and its
detection of interrupt from usermode.</td>
<td>Can rely on RCU's dyntick-idle detection.</td></tr>
<tr><th align="left"><tt>NO_HZ_IDLE</tt></th>
<td>Can rely on scheduling-clock interrupt.</td>
<td>Can rely on scheduling-clock interrupt and its
detection of interrupt from usermode.</td>
<td>Can rely on RCU's dyntick-idle detection.</td></tr>
<tr><th align="left"><tt>NO_HZ_FULL</tt></th>
<td>Can only sometimes rely on scheduling-clock interrupt.
In other cases, it is necessary to bound kernel execution
times and/or use IPIs.</td>
<td>Can rely on RCU's dyntick-idle detection.</td>
<td>Can rely on RCU's dyntick-idle detection.</td></tr>
</table>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
Why can't <tt>NO_HZ_FULL</tt> in-kernel execution rely on the
scheduling-clock interrupt, just like <tt>HZ_PERIODIC</tt>
and <tt>NO_HZ_IDLE</tt> do?
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
Because, as a performance optimization, <tt>NO_HZ_FULL</tt>
does not necessarily re-enable the scheduling-clock interrupt
on entry to each and every system call.
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
However, RCU must be reliably informed as to whether any given
CPU is currently in the idle loop, and, for <tt>NO_HZ_FULL</tt>,
also whether that CPU is executing in usermode, as discussed
<a href="#Energy Efficiency">earlier</a>.
It also requires that the scheduling-clock interrupt be enabled when
RCU needs it to be:
<ol>
<li> If a CPU is either idle or executing in usermode, and RCU believes
it is non-idle, the scheduling-clock tick had better be running.
Otherwise, you will get RCU CPU stall warnings. Or at best,
very long (11-second) grace periods, with a pointless IPI waking
the CPU from time to time.
<li> If a CPU is in a portion of the kernel that executes RCU read-side
critical sections, and RCU believes this CPU to be idle, you will get
random memory corruption. <b>DON'T DO THIS!!!</b>
<br>This is one reason to test with lockdep, which will complain
about this sort of thing.
<li> If a CPU is in a portion of the kernel that is absolutely
positively no-joking guaranteed to never execute any RCU read-side
critical sections, and RCU believes this CPU to to be idle,
no problem. This sort of thing is used by some architectures
for light-weight exception handlers, which can then avoid the
overhead of <tt>rcu_irq_enter()</tt> and <tt>rcu_irq_exit()</tt>
at exception entry and exit, respectively.
Some go further and avoid the entireties of <tt>irq_enter()</tt>
and <tt>irq_exit()</tt>.
<br>Just make very sure you are running some of your tests with
<tt>CONFIG_PROVE_RCU=y</tt>, just in case one of your code paths
was in fact joking about not doing RCU read-side critical sections.
<li> If a CPU is executing in the kernel with the scheduling-clock
interrupt disabled and RCU believes this CPU to be non-idle,
and if the CPU goes idle (from an RCU perspective) every few
jiffies, no problem. It is usually OK for there to be the
occasional gap between idle periods of up to a second or so.
<br>If the gap grows too long, you get RCU CPU stall warnings.
<li> If a CPU is either idle or executing in usermode, and RCU believes
it to be idle, of course no problem.
<li> If a CPU is executing in the kernel, the kernel code
path is passing through quiescent states at a reasonable
frequency (preferably about once per few jiffies, but the
occasional excursion to a second or so is usually OK) and the
scheduling-clock interrupt is enabled, of course no problem.
<br>If the gap between a successive pair of quiescent states grows
too long, you get RCU CPU stall warnings.
</ol>
<table>
<tr><th>&nbsp;</th></tr>
<tr><th align="left">Quick Quiz:</th></tr>
<tr><td>
But what if my driver has a hardware interrupt handler
that can run for many seconds?
I cannot invoke <tt>schedule()</tt> from an hardware
interrupt handler, after all!
</td></tr>
<tr><th align="left">Answer:</th></tr>
<tr><td bgcolor="#ffffff"><font color="ffffff">
One approach is to do <tt>rcu_irq_exit();rcu_irq_enter();</tt>
every so often.
But given that long-running interrupt handlers can cause
other problems, not least for response time, shouldn't you
work to keep your interrupt handler's runtime within reasonable
bounds?
</font></td></tr>
<tr><td>&nbsp;</td></tr>
</table>
<p>
But as long as RCU is properly informed of kernel state transitions between
in-kernel execution, usermode execution, and idle, and as long as the
scheduling-clock interrupt is enabled when RCU needs it to be, you
can rest assured that the bugs you encounter will be in some other
part of RCU or some other part of the kernel!
<h3><a name="Memory Efficiency">Memory Efficiency</a></h3>
<p>
......
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