Revision b91e1302ad9b80c174a4855533f7e3aa2873355e authored by Linus Torvalds on 27 December 2016, 19:40:38 UTC, committed by Linus Torvalds on 29 December 2016, 19:03:15 UTC
In commit 62906027091f ("mm: add PageWaiters indicating tasks are waiting for a page bit") Nick Piggin made our page locking no longer unconditionally touch the hashed page waitqueue, which not only helps performance in general, but is particularly helpful on NUMA machines where the hashed wait queues can bounce around a lot. However, the "clear lock bit atomically and then test the waiters bit" sequence turns out to be much more expensive than it needs to be, because you get a nasty stall when trying to access the same word that just got updated atomically. On architectures where locking is done with LL/SC, this would be trivial to fix with a new primitive that clears one bit and tests another atomically, but that ends up not working on x86, where the only atomic operations that return the result end up being cmpxchg and xadd. The atomic bit operations return the old value of the same bit we changed, not the value of an unrelated bit. On x86, we could put the lock bit in the high bit of the byte, and use "xadd" with that bit (where the overflow ends up not touching other bits), and look at the other bits of the result. However, an even simpler model is to just use a regular atomic "and" to clear the lock bit, and then the sign bit in eflags will indicate the resulting state of the unrelated bit #7. So by moving the PageWaiters bit up to bit #7, we can atomically clear the lock bit and test the waiters bit on x86 too. And architectures with LL/SC (which is all the usual RISC suspects), the particular bit doesn't matter, so they are fine with this approach too. This avoids the extra access to the same atomic word, and thus avoids the costly stall at page unlock time. The only downside is that the interface ends up being a bit odd and specialized: clear a bit in a byte, and test the sign bit. Nick doesn't love the resulting name of the new primitive, but I'd rather make the name be descriptive and very clear about the limitation imposed by trying to work across all relevant architectures than make it be some generic thing that doesn't make the odd semantics explicit. So this introduces the new architecture primitive clear_bit_unlock_is_negative_byte(); and adds the trivial implementation for x86. We have a generic non-optimized fallback (that just does a "clear_bit()"+"test_bit(7)" combination) which can be overridden by any architecture that can do better. According to Nick, Power has the same hickup x86 has, for example, but some other architectures may not even care. All these optimizations mean that my page locking stress-test (which is just executing a lot of small short-lived shell scripts: "make test" in the git source tree) no longer makes our page locking look horribly bad. Before all these optimizations, just the unlock_page() costs were just over 3% of all CPU overhead on "make test". After this, it's down to 0.66%, so just a quarter of the cost it used to be. (The difference on NUMA is bigger, but there this micro-optimization is likely less noticeable, since the big issue on NUMA was not the accesses to 'struct page', but the waitqueue accesses that were already removed by Nick's earlier commit). Acked-by: Nick Piggin <npiggin@gmail.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Bob Peterson <rpeterso@redhat.com> Cc: Steven Whitehouse <swhiteho@redhat.com> Cc: Andrew Lutomirski <luto@kernel.org> Cc: Andreas Gruenbacher <agruenba@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
1 parent 2d706e7
cache.c
/* AFS caching stuff
*
* Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)
*
* This program is free software; you can redistribute it and/or
* modify it under the terms of the GNU General Public License
* as published by the Free Software Foundation; either version
* 2 of the License, or (at your option) any later version.
*/
#include <linux/sched.h>
#include "internal.h"
static uint16_t afs_cell_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t buflen);
static uint16_t afs_cell_cache_get_aux(const void *cookie_netfs_data,
void *buffer, uint16_t buflen);
static enum fscache_checkaux afs_cell_cache_check_aux(void *cookie_netfs_data,
const void *buffer,
uint16_t buflen);
static uint16_t afs_vlocation_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t buflen);
static uint16_t afs_vlocation_cache_get_aux(const void *cookie_netfs_data,
void *buffer, uint16_t buflen);
static enum fscache_checkaux afs_vlocation_cache_check_aux(
void *cookie_netfs_data, const void *buffer, uint16_t buflen);
static uint16_t afs_volume_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t buflen);
static uint16_t afs_vnode_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t buflen);
static void afs_vnode_cache_get_attr(const void *cookie_netfs_data,
uint64_t *size);
static uint16_t afs_vnode_cache_get_aux(const void *cookie_netfs_data,
void *buffer, uint16_t buflen);
static enum fscache_checkaux afs_vnode_cache_check_aux(void *cookie_netfs_data,
const void *buffer,
uint16_t buflen);
static void afs_vnode_cache_now_uncached(void *cookie_netfs_data);
struct fscache_netfs afs_cache_netfs = {
.name = "afs",
.version = 0,
};
struct fscache_cookie_def afs_cell_cache_index_def = {
.name = "AFS.cell",
.type = FSCACHE_COOKIE_TYPE_INDEX,
.get_key = afs_cell_cache_get_key,
.get_aux = afs_cell_cache_get_aux,
.check_aux = afs_cell_cache_check_aux,
};
struct fscache_cookie_def afs_vlocation_cache_index_def = {
.name = "AFS.vldb",
.type = FSCACHE_COOKIE_TYPE_INDEX,
.get_key = afs_vlocation_cache_get_key,
.get_aux = afs_vlocation_cache_get_aux,
.check_aux = afs_vlocation_cache_check_aux,
};
struct fscache_cookie_def afs_volume_cache_index_def = {
.name = "AFS.volume",
.type = FSCACHE_COOKIE_TYPE_INDEX,
.get_key = afs_volume_cache_get_key,
};
struct fscache_cookie_def afs_vnode_cache_index_def = {
.name = "AFS.vnode",
.type = FSCACHE_COOKIE_TYPE_DATAFILE,
.get_key = afs_vnode_cache_get_key,
.get_attr = afs_vnode_cache_get_attr,
.get_aux = afs_vnode_cache_get_aux,
.check_aux = afs_vnode_cache_check_aux,
.now_uncached = afs_vnode_cache_now_uncached,
};
/*
* set the key for the index entry
*/
static uint16_t afs_cell_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t bufmax)
{
const struct afs_cell *cell = cookie_netfs_data;
uint16_t klen;
_enter("%p,%p,%u", cell, buffer, bufmax);
klen = strlen(cell->name);
if (klen > bufmax)
return 0;
memcpy(buffer, cell->name, klen);
return klen;
}
/*
* provide new auxiliary cache data
*/
static uint16_t afs_cell_cache_get_aux(const void *cookie_netfs_data,
void *buffer, uint16_t bufmax)
{
const struct afs_cell *cell = cookie_netfs_data;
uint16_t dlen;
_enter("%p,%p,%u", cell, buffer, bufmax);
dlen = cell->vl_naddrs * sizeof(cell->vl_addrs[0]);
dlen = min(dlen, bufmax);
dlen &= ~(sizeof(cell->vl_addrs[0]) - 1);
memcpy(buffer, cell->vl_addrs, dlen);
return dlen;
}
/*
* check that the auxiliary data indicates that the entry is still valid
*/
static enum fscache_checkaux afs_cell_cache_check_aux(void *cookie_netfs_data,
const void *buffer,
uint16_t buflen)
{
_leave(" = OKAY");
return FSCACHE_CHECKAUX_OKAY;
}
/*****************************************************************************/
/*
* set the key for the index entry
*/
static uint16_t afs_vlocation_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t bufmax)
{
const struct afs_vlocation *vlocation = cookie_netfs_data;
uint16_t klen;
_enter("{%s},%p,%u", vlocation->vldb.name, buffer, bufmax);
klen = strnlen(vlocation->vldb.name, sizeof(vlocation->vldb.name));
if (klen > bufmax)
return 0;
memcpy(buffer, vlocation->vldb.name, klen);
_leave(" = %u", klen);
return klen;
}
/*
* provide new auxiliary cache data
*/
static uint16_t afs_vlocation_cache_get_aux(const void *cookie_netfs_data,
void *buffer, uint16_t bufmax)
{
const struct afs_vlocation *vlocation = cookie_netfs_data;
uint16_t dlen;
_enter("{%s},%p,%u", vlocation->vldb.name, buffer, bufmax);
dlen = sizeof(struct afs_cache_vlocation);
dlen -= offsetof(struct afs_cache_vlocation, nservers);
if (dlen > bufmax)
return 0;
memcpy(buffer, (uint8_t *)&vlocation->vldb.nservers, dlen);
_leave(" = %u", dlen);
return dlen;
}
/*
* check that the auxiliary data indicates that the entry is still valid
*/
static
enum fscache_checkaux afs_vlocation_cache_check_aux(void *cookie_netfs_data,
const void *buffer,
uint16_t buflen)
{
const struct afs_cache_vlocation *cvldb;
struct afs_vlocation *vlocation = cookie_netfs_data;
uint16_t dlen;
_enter("{%s},%p,%u", vlocation->vldb.name, buffer, buflen);
/* check the size of the data is what we're expecting */
dlen = sizeof(struct afs_cache_vlocation);
dlen -= offsetof(struct afs_cache_vlocation, nservers);
if (dlen != buflen)
return FSCACHE_CHECKAUX_OBSOLETE;
cvldb = container_of(buffer, struct afs_cache_vlocation, nservers);
/* if what's on disk is more valid than what's in memory, then use the
* VL record from the cache */
if (!vlocation->valid || vlocation->vldb.rtime == cvldb->rtime) {
memcpy((uint8_t *)&vlocation->vldb.nservers, buffer, dlen);
vlocation->valid = 1;
_leave(" = SUCCESS [c->m]");
return FSCACHE_CHECKAUX_OKAY;
}
/* need to update the cache if the cached info differs */
if (memcmp(&vlocation->vldb, buffer, dlen) != 0) {
/* delete if the volume IDs for this name differ */
if (memcmp(&vlocation->vldb.vid, &cvldb->vid,
sizeof(cvldb->vid)) != 0
) {
_leave(" = OBSOLETE");
return FSCACHE_CHECKAUX_OBSOLETE;
}
_leave(" = UPDATE");
return FSCACHE_CHECKAUX_NEEDS_UPDATE;
}
_leave(" = OKAY");
return FSCACHE_CHECKAUX_OKAY;
}
/*****************************************************************************/
/*
* set the key for the volume index entry
*/
static uint16_t afs_volume_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t bufmax)
{
const struct afs_volume *volume = cookie_netfs_data;
uint16_t klen;
_enter("{%u},%p,%u", volume->type, buffer, bufmax);
klen = sizeof(volume->type);
if (klen > bufmax)
return 0;
memcpy(buffer, &volume->type, sizeof(volume->type));
_leave(" = %u", klen);
return klen;
}
/*****************************************************************************/
/*
* set the key for the index entry
*/
static uint16_t afs_vnode_cache_get_key(const void *cookie_netfs_data,
void *buffer, uint16_t bufmax)
{
const struct afs_vnode *vnode = cookie_netfs_data;
uint16_t klen;
_enter("{%x,%x,%llx},%p,%u",
vnode->fid.vnode, vnode->fid.unique, vnode->status.data_version,
buffer, bufmax);
klen = sizeof(vnode->fid.vnode);
if (klen > bufmax)
return 0;
memcpy(buffer, &vnode->fid.vnode, sizeof(vnode->fid.vnode));
_leave(" = %u", klen);
return klen;
}
/*
* provide updated file attributes
*/
static void afs_vnode_cache_get_attr(const void *cookie_netfs_data,
uint64_t *size)
{
const struct afs_vnode *vnode = cookie_netfs_data;
_enter("{%x,%x,%llx},",
vnode->fid.vnode, vnode->fid.unique,
vnode->status.data_version);
*size = vnode->status.size;
}
/*
* provide new auxiliary cache data
*/
static uint16_t afs_vnode_cache_get_aux(const void *cookie_netfs_data,
void *buffer, uint16_t bufmax)
{
const struct afs_vnode *vnode = cookie_netfs_data;
uint16_t dlen;
_enter("{%x,%x,%Lx},%p,%u",
vnode->fid.vnode, vnode->fid.unique, vnode->status.data_version,
buffer, bufmax);
dlen = sizeof(vnode->fid.unique) + sizeof(vnode->status.data_version);
if (dlen > bufmax)
return 0;
memcpy(buffer, &vnode->fid.unique, sizeof(vnode->fid.unique));
buffer += sizeof(vnode->fid.unique);
memcpy(buffer, &vnode->status.data_version,
sizeof(vnode->status.data_version));
_leave(" = %u", dlen);
return dlen;
}
/*
* check that the auxiliary data indicates that the entry is still valid
*/
static enum fscache_checkaux afs_vnode_cache_check_aux(void *cookie_netfs_data,
const void *buffer,
uint16_t buflen)
{
struct afs_vnode *vnode = cookie_netfs_data;
uint16_t dlen;
_enter("{%x,%x,%llx},%p,%u",
vnode->fid.vnode, vnode->fid.unique, vnode->status.data_version,
buffer, buflen);
/* check the size of the data is what we're expecting */
dlen = sizeof(vnode->fid.unique) + sizeof(vnode->status.data_version);
if (dlen != buflen) {
_leave(" = OBSOLETE [len %hx != %hx]", dlen, buflen);
return FSCACHE_CHECKAUX_OBSOLETE;
}
if (memcmp(buffer,
&vnode->fid.unique,
sizeof(vnode->fid.unique)
) != 0) {
unsigned unique;
memcpy(&unique, buffer, sizeof(unique));
_leave(" = OBSOLETE [uniq %x != %x]",
unique, vnode->fid.unique);
return FSCACHE_CHECKAUX_OBSOLETE;
}
if (memcmp(buffer + sizeof(vnode->fid.unique),
&vnode->status.data_version,
sizeof(vnode->status.data_version)
) != 0) {
afs_dataversion_t version;
memcpy(&version, buffer + sizeof(vnode->fid.unique),
sizeof(version));
_leave(" = OBSOLETE [vers %llx != %llx]",
version, vnode->status.data_version);
return FSCACHE_CHECKAUX_OBSOLETE;
}
_leave(" = SUCCESS");
return FSCACHE_CHECKAUX_OKAY;
}
/*
* indication the cookie is no longer uncached
* - this function is called when the backing store currently caching a cookie
* is removed
* - the netfs should use this to clean up any markers indicating cached pages
* - this is mandatory for any object that may have data
*/
static void afs_vnode_cache_now_uncached(void *cookie_netfs_data)
{
struct afs_vnode *vnode = cookie_netfs_data;
struct pagevec pvec;
pgoff_t first;
int loop, nr_pages;
_enter("{%x,%x,%Lx}",
vnode->fid.vnode, vnode->fid.unique, vnode->status.data_version);
pagevec_init(&pvec, 0);
first = 0;
for (;;) {
/* grab a bunch of pages to clean */
nr_pages = pagevec_lookup(&pvec, vnode->vfs_inode.i_mapping,
first,
PAGEVEC_SIZE - pagevec_count(&pvec));
if (!nr_pages)
break;
for (loop = 0; loop < nr_pages; loop++)
ClearPageFsCache(pvec.pages[loop]);
first = pvec.pages[nr_pages - 1]->index + 1;
pvec.nr = nr_pages;
pagevec_release(&pvec);
cond_resched();
}
_leave("");
}
![swh spinner](/static/img/swh-spinner.gif)
Computing file changes ...