Merge pull request #146 from sched-ext/htejun

tools/sched_ext: Bring back scx_central and scx_flatcg and simplify README

tools/sched_ext/Makefile

@@ -176,7 +176,7 @@ $(INCLUDE_DIR)/%.bpf.skel.h: $(SCXOBJ_DIR)/%.bpf.o $(INCLUDE_DIR)/vmlinux.h $(BP
 
 SCX_COMMON_DEPS := include/scx/common.h include/scx/user_exit_info.h | $(BINDIR)
 
-c-sched-targets = scx_simple scx_qmap
+c-sched-targets = scx_simple scx_qmap scx_central scx_flatcg
 
 $(addprefix $(BINDIR)/,$(c-sched-targets)): \
 	$(BINDIR)/%: \

tools/sched_ext/README.md

@@ -154,55 +154,56 @@ a full vmlinux.h file, you can compile the schedulers using `make`:
 $ make -j($nproc)
 ```
 
-# Schedulers
+# Example schedulers
 
-This directory contains the following simple schedulers as examples. For
-more, visit https://github.com/sched-ext/scx.
+This directory contains the following example schedulers. These schedulers are
+for testing and demonstrating different aspects of sched_ext. While some may be
+useful in limited scenarios, they are not intended to be practical.
 
---------------------------------------------------------------------------------
+For more scheduler implementations, tools and documentation, visit
+https://github.com/sched-ext/scx.
 
 ## scx_simple
 
-### Overview
+A simple scheduler that provides an example of a minimal sched_ext scheduler.
+scx_simple can be run in either global weighted vtime mode, or FIFO mode.
 
-A simple scheduler that provides an example of a minimal sched_ext
-scheduler. scx_simple can be run in either global weighted vtime mode, or
-FIFO mode.
-
-### Typical Use Case
-
-Though very simple, this scheduler should perform reasonably well on
-single-socket CPUs with a uniform L3 cache topology. Note that while running in
-global FIFO mode may work well for some workloads, saturating threads can
-easily drown out inactive ones.
-
-### Production Ready?
-
-This scheduler could be used in a production environment, assuming the hardware
-constraints enumerated above, and assuming the workload can accommodate a
-simple scheduling policy.
-
---------------------------------------------------------------------------------
+Though very simple, in limited scenarios, this scheduler can perform reasonably
+well on single-socket systems with a unified L3 cache.
 
 ## scx_qmap
 
-### Overview
-
 Another simple, yet slightly more complex scheduler that provides an example of
 a basic weighted FIFO queuing policy. It also provides examples of some common
 useful BPF features, such as sleepable per-task storage allocation in the
 `ops.prep_enable()` callback, and using the `BPF_MAP_TYPE_QUEUE` map type to
 enqueue tasks. It also illustrates how core-sched support could be implemented.
 
-### Typical Use Case
+## scx_central
+
+A "central" scheduler where scheduling decisions are made from a single CPU.
+This scheduler illustrates how scheduling decisions can be dispatched from a
+single CPU, allowing other cores to run with infinite slices, without timer
+ticks, and without having to incur the overhead of making scheduling decisions.
+
+The approach demonstrated by this scheduler may be useful for any workload that
+benefits from minimizing scheduling overhead and timer ticks. An example of
+where this could be particularly useful is running VMs, where running with
+infinite slices and no timer ticks allows the VM to avoid unnecessary expensive
+vmexits.
 
-Purely used to illustrate sched_ext features.
+## scx_flatcg
 
-### Production Ready?
+A flattened cgroup hierarchy scheduler. This scheduler implements hierarchical
+weight-based cgroup CPU control by flattening the cgroup hierarchy into a single
+layer, by compounding the active weight share at each level. The effect of this
+is a much more performant CPU controller, which does not need to descend down
+cgroup trees in order to properly compute a cgroup's share.
 
-No
+Similar to scx_simple, in limited scenarios, this scheduler can perform
+reasonably well on single socket-socket systems with a unified L3 cache and show
+significantly lowered hierarchical scheduling overhead.
 
---------------------------------------------------------------------------------
 
 # Troubleshooting