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Allow counters_read(9) to take an optional scratch buffer.

Using a scratch buffer makes it possible to take a consistent snapshot of
per-CPU counters without having to allocate memory.

Makes ddb(4) show uvmexp command work in OOM situations.

ok kn@, mvs@, cheloha@

.\"	$OpenBSD: counters_alloc.9,v 1.13 2023/09/16 09:33:28 mpi Exp $
.\"
.\" Copyright (c) 2016 David Gwynne <dlg@openbsd.org>
.\"
.\" Permission to use, copy, modify, and distribute this software for any
.\" purpose with or without fee is hereby granted, provided that the above
.\" copyright notice and this permission notice appear in all copies.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
.\"
.Dd $Mdocdate: September 16 2023 $
.Dt COUNTERS_ALLOC 9
.Os
.Sh NAME
.Nm counters_alloc ,
.Nm counters_free ,
.Nm COUNTERS_BOOT_MEMORY ,
.Nm COUNTERS_BOOT_INITIALIZER ,
.Nm counters_alloc_ncpus ,
.Nm counters_enter ,
.Nm counters_leave ,
.Nm counters_inc ,
.Nm counters_add ,
.Nm counters_pkt ,
.Nm counters_read ,
.Nm counters_zero
.Nd per CPU counters
.Sh SYNOPSIS
.In sys/percpu.h
.Ft struct cpumem *
.Fn counters_alloc "unsigned int ncounters"
.Ft void
.Fn counters_free "struct cpumem *cm" "unsigned int ncounters"
.Fn COUNTERS_BOOT_MEMORY "NAME" "unsigned int ncounters"
.Fn COUNTERS_BOOT_INITIALIZER "NAME"
.Ft struct cpumemt *
.Fo counters_alloc_ncpus
.Fa "struct cpumem *cm"
.Fa "unsigned int ncounters"
.Fc
.Ft uint64_t *
.Fn counters_enter "struct counters_ref *ref" "struct cpumem *cm"
.Ft void
.Fn counters_leave "struct counters_ref *ref" "struct cpumem *cm"
.Ft void
.Fn counters_inc "struct cpumem *cm" "unsigned int counter"
.Ft void
.Fn counters_add "struct cpumem *cm" "unsigned int counter" "uint64_t v"
.Ft void
.Fo counters_pkt
.Fa "struct cpumem *cm"
.Fa "unsigned int pcounter"
.Fa "unsigned int bcounter"
.Fa "uint64_t bytes"
.Fc
.Ft void
.Fo counters_read
.Fa "struct cpumem *cm"
.Fa "uint64_t *counters"
.Fa "unsigned int ncounters"
.Fa "uint64_t *scratch"
.Fc
.Ft void
.Fn counters_zero "struct cpumem *cm" "unsigned int ncounters"
.Sh DESCRIPTION
The per CPU counter API builds on the per CPU memory API and provides
access to a series of uint64_t counter values on each CPU.
.Pp
The API provides versioning of counter updates without using
interlocked CPU instructions so they can all be read in a consistent
state.
Updates to counters should be limited to addition or subtraction
of uint64_t values.
.Pp
An alternate implementation of the API is provided on uni-processor
systems
(i.e. when the kernel is not built with
.Dv MULTIPROCESSOR
defined)
that provides no overhead compared to direct access to a
data structure.
This allows the API to be used without affecting the performance
of uni-processor systems.
.Pp
.Fn counters_alloc
allocates memory for a series of uint64_t values on each CPU.
.Fa ncounters
specifies the number of counters to be allocated.
The counters will be zeroed on allocation.
.Pp
.Fn counters_free
deallocates each CPU's counters.
The same
.Fa ncounters
argument originally provided to
.Fn counters_alloc
must be passed to
.Fn counters_free .
.Pp
.Fn counters_alloc
may only be used after all the CPUs in the system have been attached.
If a set of CPU counters needs to be available during early boot,
a cpumem pointer and counters for the boot CPU may be statically
allocated.
.Pp
.Fn COUNTERS_BOOT_MEMORY
statically allocates a set of counter for use on the boot CPU
before the other CPUs in the system have been attached.
The allocation is identified by
.Fa NAME
and provides memory for the number of counters specified by
.Fa ncounters .
The counters will be initialised to zero.
.Pp
.Fn COUNTERS_BOOT_INITIALIZER
is used to initialise a cpumem pointer with the memory that was previously
allocated using
.Fn COUNTERS_BOOT_MEMORY
and identified by
.Fa NAME .
.Pp
.Fn counters_alloc_ncpus
allocates additional sets of counters for the CPUs that were attached
during boot.
The cpumem structure
.Fa cm
must have been initialised with
.Fn COUNTERS_BOOT_INITIALIZER .
The same number of counters originally passed to
.Fa COUNTERS_BOOT_MEMORY
must be specified by
.Fa ncounters .
The counters on the boot CPU will be preserved, while the counters
for the additional CPUs will be zeroed on allocation.
.Pp
Counters that have been allocated with
.Fn COUNTERS_BOOT_MEMORY
and
.Fn counters_alloc_ncpus
cannot be deallocated with
.Fa counters_free .
.Pp
.Fn counters_enter
provides access to the current CPU's set of counters referenced by
.Fa cm .
The caller's reference to the counters is held by
.Fa ref .
.Pp
.Fn counters_leave
indicates the end of access to the current CPU's set of counters referenced by
.Fa cm .
The reference held by
.Fa ref
is released by this call.
.Pp
.Fn counters_inc
increments the counter at the index specified by
.Fa counter
in the array of counters referenced by
.Fa cm .
.Pp
.Fn counters_add
adds the value of
.Fa v
to the counter at the index specified by
.Fa counter
in the array of counters referenced by
.Fa cm .
.Pp
.Fn counters_pkt
increments the value at the index specified by
.Fa pcounter
and adds the value of
.Fa bytes
to the counter at the index specified by
.Fa bcounter
in the array of counters referenced by
.Fa cm .
.Pp
.Fn counters_read
iterates over each CPU's set of counters referenced by
.Fa cm ,
takes a consistent snapshot of the counters, and then sums them together.
The sum of the counters is written to the
.Fa counters
array.
The number of counters is specified with
.Fa ncounters .
.Fa scratch
may point to a buffer used to temporarily hold
.Fa counters .
If
.Dv NULL ,
one will be dynamically allocated and freed.
.Pp
.Fn counters_zero
iterates over each CPU's set of counters referenced by
.Fa cm
and zeroes them.
The number of counters is specified with
.Fa ncounters .
.Fn counters_zero
itself does not prevent concurrent updates of the counters; it is
up to the caller to serialise this call with other actions.
.Sh CONTEXT
.Fn counters_alloc ,
.Fn counters_free ,
.Fn counters_alloc_ncpus ,
and
.Fn counters_read
may be called during autoconf, or from process context.
.Pp
.Fn counters_enter ,
.Fn counters_leave ,
.Fn counters_inc ,
.Fn counters_add ,
.Fn counters_pkt ,
and
.Fn counters_zero
may be called during autoconf, from process context, or from interrupt
context.
The per CPU counters API does not provide any locking or serialisation
of access to each CPU's set of counters beyond isolating each CPU's
update.
It is up to the caller to provide appropriate locking or serialisation
around calls to these functions to prevent concurrent access to the
relevant data structures.
.Sh RETURN VALUES
.Fn counters_alloc
and
.Fn counters_alloc_ncpus
will return an opaque cpumem pointer that references each CPU's
set of counters.
.Pp
.Fn counters_enter
returns a reference to the current CPU's set of counters.
.Sh EXAMPLES
The following is an example of providing per CPU counters at boot
time based on the
.Xr mbuf 9
statistics code in
.Pa sys/kern/uipc_mbuf.c .
.Bd -literal
/* mbuf stats */
COUNTERS_BOOT_MEMORY(mbstat_boot, MBSTAT_COUNT);
struct cpumem *mbstat = COUNTERS_BOOT_INITIALIZER(mbstat_boot);

/*
 * this function is called from init_main.c after devices
 * (including additional CPUs) have been attached
 */
void
mbcpuinit()
{
	mbstat = counters_alloc_ncpus(mbstat, MBSTAT_COUNT);
}

struct mbuf *
m_get(int nowait, int type)
{
	...

        struct counters_ref cr;
        uint64_t *counters;
        int s;

	...

        s = splnet();
        counters = counters_enter(&cr, mbstat);
        counters[type]++;
        counters_leave(&cr, mbstat);
        splx(s);

	...
}

struct mbuf *
m_free(struct mbuf *m)
{
	...

        struct counters_ref cr;
        uint64_t *counters;
        int s;

	...

        s = splnet();
        counters = counters_enter(&cr, mbstat);
        counters[m->m_type]--;
        counters_leave(&cr, mbstat);
        splx(s);

	...
}
.Ed
.Sh SEE ALSO
.Xr cpumem_get 9 ,
.Xr malloc 9
.Sh HISTORY
The per CPU counter API first appeared in
.Ox 6.1 .
.Sh AUTHORS
The per CPU counter API was written by
.An David Gwynne Aq Mt dlg@openbsd.org .