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freebsd-nq/share/man/man9/zone.9
Jonathan T. Looney 54503a13d8 Add a safety net to reclaim mbufs when one of the mbuf zones become 
exhausted.

It is possible for a bug in the code (or, theoretically, even unusual
network conditions) to exhaust all possible mbufs or mbuf clusters.
When this occurs, things can grind to a halt fairly quickly. However,
we currently do not call mb_reclaim() unless the entire system is
experiencing a low-memory condition.

While it is best to try to prevent exhaustion of one of the mbuf zones,
it would also be useful to have a mechanism to attempt to recover from
these situations by freeing "expendable" mbufs.

This patch makes two changes:

a) The patch adds a generic API to the UMA zone allocator to set a
function that should be called when an allocation fails because the
zone limit has been reached. Because of the way this function can be
called, it really should do minimal work.

b) The patch uses this API to try to free mbufs when an allocation
fails from one of the mbuf zones because the zone limit has been
reached. The function schedules a callout to run mb_reclaim().

Differential Revision:	https://reviews.freebsd.org/D3864
Reviewed by:	gnn
Comments by:	rrs, glebius
MFC after:	2 weeks
Sponsored by:	Juniper Networks
2015-12-20 02:05:33 +00:00

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.\"-
.\" Copyright (c) 2001 Dag-Erling Coïdan Smørgrav
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.\" $FreeBSD$
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.Dd December 20, 2015
.Dt ZONE 9
.Os
.Sh NAME
.Nm uma_zcreate ,
.Nm uma_zalloc ,
.Nm uma_zalloc_arg ,
.Nm uma_zfree ,
.Nm uma_zfree_arg ,
.Nm uma_find_refcnt ,
.Nm uma_zdestroy ,
.Nm uma_zone_set_max,
.Nm uma_zone_get_max,
.Nm uma_zone_get_cur,
.Nm uma_zone_set_warning,
.Nm uma_zone_set_maxaction
.Nd zone allocator
.Sh SYNOPSIS
.In sys/param.h
.In sys/queue.h
.In vm/uma.h
.Ft uma_zone_t
.Fo uma_zcreate
.Fa "char *name" "int size"
.Fa "uma_ctor ctor" "uma_dtor dtor" "uma_init uminit" "uma_fini fini"
.Fa "int align" "uint16_t flags"
.Fc
.Ft "void *"
.Fn uma_zalloc "uma_zone_t zone" "int flags"
.Ft "void *"
.Fn uma_zalloc_arg "uma_zone_t zone" "void *arg" "int flags"
.Ft void
.Fn uma_zfree "uma_zone_t zone" "void *item"
.Ft void
.Fn uma_zfree_arg "uma_zone_t zone" "void *item" "void *arg"
.Ft "uint32_t *"
.Fn uma_find_refcnt "uma_zone_t zone" "void *item"
.Ft void
.Fn uma_zdestroy "uma_zone_t zone"
.Ft int
.Fn uma_zone_set_max "uma_zone_t zone" "int nitems"
.Ft int
.Fn uma_zone_get_max "uma_zone_t zone"
.Ft int
.Fn uma_zone_get_cur "uma_zone_t zone"
.Ft void
.Fn uma_zone_set_warning "uma_zone_t zone" "const char *warning"
.Ft void
.Fn uma_zone_set_maxaction "uma_zone_t zone" "void (*maxaction)(uma_zone_t)"
.In sys/sysctl.h
.Fn SYSCTL_UMA_MAX parent nbr name access zone descr
.Fn SYSCTL_ADD_UMA_MAX ctx parent nbr name access zone descr
.Fn SYSCTL_UMA_CUR parent nbr name access zone descr
.Fn SYSCTL_ADD_UMA_CUR ctx parent nbr name access zone descr
.Sh DESCRIPTION
The zone allocator provides an efficient interface for managing
dynamically-sized collections of items of similar size.
The zone allocator can work with preallocated zones as well as with
runtime-allocated ones, and is therefore available much earlier in the
boot process than other memory management routines.
.Pp
A zone is an extensible collection of items of identical size.
The zone allocator keeps track of which items are in use and which
are not, and provides functions for allocating items from the zone and
for releasing them back (which makes them available for later use).
.Pp
After the first allocation of an item,
it will have been cleared to zeroes, however subsequent allocations
will retain the contents as of the last free.
.Pp
The
.Fn uma_zcreate
function creates a new zone from which items may then be allocated from.
The
.Fa name
argument is a text name of the zone for debugging and stats; this memory
should not be freed until the zone has been deallocated.
.Pp
The
.Fa ctor
and
.Fa dtor
arguments are callback functions that are called by
the uma subsystem at the time of the call to
.Fn uma_zalloc
and
.Fn uma_zfree
respectively.
Their purpose is to provide hooks for initializing or
destroying things that need to be done at the time of the allocation
or release of a resource.
A good usage for the
.Fa ctor
and
.Fa dtor
callbacks
might be to adjust a global count of the number of objects allocated.
.Pp
The
.Fa uminit
and
.Fa fini
arguments are used to optimize the allocation of
objects from the zone.
They are called by the uma subsystem whenever
it needs to allocate or free several items to satisfy requests or memory
pressure.
A good use for the
.Fa uminit
and
.Fa fini
callbacks might be to
initialize and destroy mutexes contained within the object.
This would
allow one to re-use already initialized mutexes when an object is returned
from the uma subsystem's object cache.
They are not called on each call to
.Fn uma_zalloc
and
.Fn uma_zfree
but rather in a batch mode on several objects.
.Pp
The
.Fa flags
argument of the
.Fn uma_zcreate
is a subset of the following flags:
.Bl -tag -width "foo"
.It Dv UMA_ZONE_NOFREE
Slabs of the zone are never returned back to VM.
.It Dv UMA_ZONE_REFCNT
Each item in the zone would have internal reference counter associated with it.
See
.Fn uma_find_refcnt .
.It Dv UMA_ZONE_NODUMP
Pages belonging to the zone will not be included into mini-dumps.
.It Dv UMA_ZONE_PCPU
An allocation from zone would have
.Va mp_ncpu
shadow copies, that are privately assigned to CPUs.
A CPU can address its private copy using base allocation address plus
multiple of current CPU id and
.Fn sizeof "struct pcpu" :
.Bd -literal -offset indent
foo_zone = uma_zcreate(..., UMA_ZONE_PCPU);
...
foo_base = uma_zalloc(foo_zone, ...);
...
critical_enter();
foo_pcpu = (foo_t *)zpcpu_get(foo_base);
/* do something with foo_pcpu */
critical_exit();
.Ed
.It Dv UMA_ZONE_OFFPAGE
By default book-keeping of items within a slab is done in the slab page itself.
This flag explicitly tells subsystem that book-keeping structure should be
allocated separately from special internal zone.
This flag requires either
.Dv UMA_ZONE_VTOSLAB
or
.Dv UMA_ZONE_HASH ,
since subsystem requires a mechanism to find a book-keeping structure
to an item beeing freed.
The subsystem may choose to prefer offpage book-keeping for certain zones
implicitly.
.It Dv UMA_ZONE_ZINIT
The zone will have its
.Ft uma_init
method set to internal method that initializes a new allocated slab
to all zeros.
Do not mistake
.Ft uma_init
method with
.Ft uma_ctor .
A zone with
.Dv UMA_ZONE_ZINIT
flag would not return zeroed memory on every
.Fn uma_zalloc .
.It Dv UMA_ZONE_HASH
The zone should use an internal hash table to find slab book-keeping
structure where an allocation being freed belongs to.
.It Dv UMA_ZONE_VTOSLAB
The zone should use special field of
.Vt vm_page_t
to find slab book-keeping structure where an allocation being freed belongs to.
.It Dv UMA_ZONE_MALLOC
The zone is for the
.Xr malloc 9
subsystem.
.It Dv UMA_ZONE_VM
The zone is for the VM subsystem.
.El
.Pp
To allocate an item from a zone, simply call
.Fn uma_zalloc
with a pointer to that zone
and set the
.Fa flags
argument to selected flags as documented in
.Xr malloc 9 .
It will return a pointer to an item if successful,
or
.Dv NULL
in the rare case where all items in the zone are in use and the
allocator is unable to grow the zone
and
.Dv M_NOWAIT
is specified.
.Pp
Items are released back to the zone from which they were allocated by
calling
.Fn uma_zfree
with a pointer to the zone and a pointer to the item.
If
.Fa item
is
.Dv NULL ,
then
.Fn uma_zfree
does nothing.
.Pp
The variations
.Fn uma_zalloc_arg
and
.Fn uma_zfree_arg
allow to
specify an argument for the
.Dv ctor
and
.Dv dtor
functions, respectively.
.Pp
If zone was created with
.Dv UMA_ZONE_REFCNT
flag, then pointer to reference counter for an item can be retrieved with
help of the
.Fn uma_find_refcnt
function.
.Pp
Created zones,
which are empty,
can be destroyed using
.Fn uma_zdestroy ,
freeing all memory that was allocated for the zone.
All items allocated from the zone with
.Fn uma_zalloc
must have been freed with
.Fn uma_zfree
before.
.Pp
The
.Fn uma_zone_set_max
function limits the number of items
.Pq and therefore memory
that can be allocated to
.Fa zone .
The
.Fa nitems
argument specifies the requested upper limit number of items.
The effective limit is returned to the caller, as it may end up being higher
than requested due to the implementation rounding up to ensure all memory pages
allocated to the zone are utilised to capacity.
The limit applies to the total number of items in the zone, which includes
allocated items, free items and free items in the per-cpu caches.
On systems with more than one CPU it may not be possible to allocate
the specified number of items even when there is no shortage of memory,
because all of the remaining free items may be in the caches of the
other CPUs when the limit is hit.
.Pp
The
.Fn uma_zone_get_max
function returns the effective upper limit number of items for a zone.
.Pp
The
.Fn uma_zone_get_cur
function returns the approximate current occupancy of the zone.
The returned value is approximate because appropriate synchronisation to
determine an exact value is not performed by the implementation.
This ensures low overhead at the expense of potentially stale data being used
in the calculation.
.Pp
The
.Fn uma_zone_set_warning
function sets a warning that will be printed on the system console when the
given zone becomes full and fails to allocate an item.
The warning will be printed no more often than every five minutes.
Warnings can be turned off globally by setting the
.Va vm.zone_warnings
sysctl tunable to
.Va 0 .
.Pp
The
.Fn uma_zone_set_maxaction
function sets a function that will be called when the given zone becomes full
and fails to allocate an item.
The function will be called with the zone locked. Also, the function
that called the allocation function may have held additional locks. Therefore,
this function should do very little work (similar to a signal handler).
.Pp
The
.Fn SYSCTL_UMA_MAX parent nbr name access zone descr
macro declares a static
.Xr sysctl
oid that exports the effective upper limit number of items for a zone.
The
.Fa zone
argument should be a pointer to
.Vt uma_zone_t .
A read of the oid returns value obtained through
.Fn uma_zone_get_max .
A write to the oid sets new value via
.Fn uma_zone_set_max .
The
.Fn SYSCTL_ADD_UMA_MAX ctx parent nbr name access zone descr
macro is provided to create this type of oid dynamically.
.Pp
The
.Fn SYSCTL_UMA_CUR parent nbr name access zone descr
macro declares a static read only
.Xr sysctl
oid that exports the approximate current occupancy of the zone.
The
.Fa zone
argument should be a pointer to
.Vt uma_zone_t .
A read of the oid returns value obtained through
.Fn uma_zone_get_cur .
The
.Fn SYSCTL_ADD_UMA_CUR ctx parent nbr name zone descr
macro is provided to create this type of oid dynamically.
.Sh RETURN VALUES
The
.Fn uma_zalloc
function returns a pointer to an item, or
.Dv NULL
if the zone ran out of unused items
and
.Dv M_NOWAIT
was specified.
.Sh SEE ALSO
.Xr malloc 9
.Sh HISTORY
The zone allocator first appeared in
.Fx 3.0 .
It was radically changed in
.Fx 5.0
to function as a slab allocator.
.Sh AUTHORS
.An -nosplit
The zone allocator was written by
.An John S. Dyson .
The zone allocator was rewritten in large parts by
.An Jeff Roberson Aq Mt jeff@FreeBSD.org
to function as a slab allocator.
.Pp
This manual page was written by
.An Dag-Erling Sm\(/orgrav Aq Mt des@FreeBSD.org .
Changes for UMA by
.An Jeroen Ruigrok van der Werven Aq Mt asmodai@FreeBSD.org .
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