Re: [PATCH 6/8] lib: bitmap: support "N" as an alias for size of bitmap

From: Yury Norov
Date: Tue Feb 09 2021 - 20:31:29 EST

On Tue, Feb 9, 2021 at 3:01 PM Paul Gortmaker
<paul.gortmaker@xxxxxxxxxxxxx> wrote:
>
> 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 <yury.norov@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> Cc: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> Suggested-by: Yury Norov <yury.norov@xxxxxxxxx> # move it from CPU code
> Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
> ---
> .../admin-guide/kernel-parameters.rst | 7 +++++
> lib/bitmap.c | 27 ++++++++++++++-----
> 2 files changed, 27 insertions(+), 7 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.rst b/Documentation/admin-guide/kernel-parameters.rst
> index 682ab28b5c94..7733a773f5f8 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
> diff --git a/lib/bitmap.c b/lib/bitmap.c
> index 6b568f98af3d..cc7cb1fca1ac 100644
> --- a/lib/bitmap.c
> +++ b/lib/bitmap.c
> @@ -530,11 +530,17 @@ static int bitmap_check_region(const struct bitmap_region *br)
> return 0;
> }
>
> -static const char *bitmap_getnum(const char *str, unsigned int *num)
> +static const char *bitmap_getnum(const char *str, unsigned int *num,
> + unsigned int lastbit)

The idea of struct bitmap_region is avoid passing the lastbit to the functions.
But here you do pass. Can you please be consistent? Or if I misunderstand
the idea of struct bitmap_region, can you please clarify it?

Also, I don't think that in this specific case it's worth it to create
a hierarchy of
structures. Just adding lastbits to struct region will be simpler and more
transparent.

> {
> unsigned long long n;
> unsigned int len;
>
> + if (str[0] == 'N') {
> + *num = lastbit;
> + return str + 1;
> + }
> +
> len = _parse_integer(str, 10, &n);
> if (!len)
> return ERR_PTR(-EINVAL);
> @@ -580,9 +586,12 @@ static const char *bitmap_find_region_reverse(const char *start, const char *end
> return end;
> }
>
> -static const char *bitmap_parse_region(const char *str, struct region *r)
> +static const char *bitmap_parse_region(const char *str, struct bitmap_region *br)

It seems like you declare struct bitmap_region in patch 8, but use it
in patch 6.
Can you please reorder your patches and resubmit?

> {
> - str = bitmap_getnum(str, &r->start);
> + struct region *r = br->r;
> + unsigned int lastbit = br->nbits - 1;
> +
> + str = bitmap_getnum(str, &r->start, lastbit);
> if (IS_ERR(str))
> return str;
>
> @@ -592,7 +601,7 @@ static const char *bitmap_parse_region(const char *str, struct region *r)
> if (*str != '-')
> return ERR_PTR(-EINVAL);
>
> - str = bitmap_getnum(str + 1, &r->end);
> + str = bitmap_getnum(str + 1, &r->end, lastbit);
> if (IS_ERR(str))
> return str;
>
> @@ -602,14 +611,14 @@ static const char *bitmap_parse_region(const char *str, struct region *r)
> if (*str != ':')
> return ERR_PTR(-EINVAL);
>
> - str = bitmap_getnum(str + 1, &r->off);
> + str = bitmap_getnum(str + 1, &r->off, lastbit);
> if (IS_ERR(str))
> return str;
>
> if (*str != '/')
> return ERR_PTR(-EINVAL);
>
> - return bitmap_getnum(str + 1, &r->group_len);
> + return bitmap_getnum(str + 1, &r->group_len, lastbit);
>
> no_end:
> r->end = r->start;
> @@ -636,6 +645,10 @@ static const char *bitmap_parse_region(const char *str, struct region *r)
> * From each group will be used only defined amount of bits.
> * Syntax: range:used_size/group_size
> * Example: 0-1023:2/256 ==> 0,1,256,257,512,513,768,769
> + * The value 'N' can be used as a dynamically substituted token for the
> + * maximum allowed value; i.e (nmaskbits - 1). Keep in mind that it is
> + * dynamic, so if system changes cause the bitmap width to change, such
> + * as more cores in a CPU list, then any ranges using N will also change.
> *
> * Returns: 0 on success, -errno on invalid input strings. Error values:
> *
> @@ -660,7 +673,7 @@ int bitmap_parselist(const char *buf, unsigned long *maskp, int nmaskbits)
> if (buf == NULL)
> return 0;
>
> - buf = bitmap_parse_region(buf, &r);
> + buf = bitmap_parse_region(buf, &br);
> if (IS_ERR(buf))
> return PTR_ERR(buf);
>
> --
> 2.17.1
>