From 2c4885d24e64941702a8f81c8e83289823ba35d0 Mon Sep 17 00:00:00 2001 From: Paul Gortmaker Date: Sun, 21 Feb 2021 03:08:25 -0500 Subject: lib: bitmap: support "N" as an alias for size of bitmap While this is done for all bitmaps, the original use case in mind was for CPU masks and cpulist_parse() as described below. It seems that a common configuration is to use the 1st couple cores for housekeeping tasks. This tends to leave the remaining ones to form a pool of similarly configured cores to take on the real workload of interest to the user. So on machine A - with 32 cores, it could be 0-3 for "system" and then 4-31 being used in boot args like nohz_full=, or rcu_nocbs= as part of setting up the worker pool of CPUs. But then newer machine B is added, and it has 48 cores, and so while the 0-3 part remains unchanged, the pool setup cpu list becomes 4-47. Multiple deployment becomes easier when we can just simply replace 31 and 47 with "N" and let the system substitute in the actual number at boot; a number that it knows better than we do. Cc: Yury Norov Cc: Peter Zijlstra Cc: "Paul E. McKenney" Cc: Rasmus Villemoes Cc: Andy Shevchenko Suggested-by: Yury Norov # move it from CPU code Acked-by: Yury Norov Signed-off-by: Paul Gortmaker Signed-off-by: Paul E. McKenney --- Documentation/admin-guide/kernel-parameters.rst | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'Documentation/admin-guide/kernel-parameters.rst') diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst index 1132796a8d96..d6e3f67953a7 100644 --- a/Documentation/admin-guide/kernel-parameters.rst +++ b/Documentation/admin-guide/kernel-parameters.rst @@ -68,6 +68,13 @@ For example one can add to the command line following parameter: where the final item represents CPUs 100,101,125,126,150,151,... +The value "N" can be used to represent the numerically last CPU on the system, +i.e "foo_cpus=16-N" would be equivalent to "16-31" on a 32 core system. + +Keep in mind that "N" is dynamic, so if system changes cause the bitmap width +to change, such as less cores in the CPU list, then N and any ranges using N +will also change. Use the same on a small 4 core system, and "16-N" becomes +"16-3" and now the same boot input will be flagged as invalid (start > end). This document may not be entirely up to date and comprehensive. The command -- cgit v1.2.3