| Message ID | 87fuxh1bo8.fsf@notabene.neil.brown.name (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Fri, Jan 29, 2016 at 11:17:43AM +1100, NeilBrown wrote: > bit-spin-locks, as used for dcache hash chains, are not fair. > This is not a problem for the dcache hash table as different CPUs are > likely to access different entries in the hash table so high contention > is not expected. > However anonymous dentryies (created by NFSD) all live on a single hash > chain "s_anon" and the bitlock on this can be highly contended, resulting > in soft-lockup warnings. Just out of curiosity, because I can't recall seeing complaints about warnings, when do you see it happen? Server reboot, maybe? It should be hitting that __d_obtain_alias() case only when a filehandle lookup finds a file without a cached dentry, which is an important case to handle, but not normally what I'd expect to be the common case. Am I forgetting something? --b. > > So introduce a global (fair) spinlock and take it before grabing the > bitlock on s_anon. This provides fairness and makes the warnings go away. > > We could alternately use s_inode_list_lock, or add another spinlock > to struct super_block. Suggestions? > > Signed-off-by: NeilBrown <neilb@suse.com> > --- > > Dave: I'd guess you would be against using the new s_inode_list_lock > for this because it is already highly contended - yes? > Is it worth adding another spinlock to 'struct super_block' ? > > Thanks, > NeilBrown > > > fs/dcache.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/fs/dcache.c b/fs/dcache.c > index 92d5140de851..e83f1ac1540c 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -104,6 +104,8 @@ static unsigned int d_hash_shift __read_mostly; > > static struct hlist_bl_head *dentry_hashtable __read_mostly; > > +static DEFINE_SPINLOCK(s_anon_lock); > + > static inline struct hlist_bl_head *d_hash(const struct dentry *parent, > unsigned int hash) > { > @@ -490,10 +492,14 @@ void __d_drop(struct dentry *dentry) > else > b = d_hash(dentry->d_parent, dentry->d_name.hash); > > + if (b == &dentry->d_sb->s_anon) > + spin_lock(&s_anon_lock); > hlist_bl_lock(b); > __hlist_bl_del(&dentry->d_hash); > dentry->d_hash.pprev = NULL; > hlist_bl_unlock(b); > + if (b == &dentry->d_sb->s_anon) > + spin_unlock(&s_anon_lock); > dentry_rcuwalk_invalidate(dentry); > } > } > @@ -1978,9 +1984,11 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected) > spin_lock(&tmp->d_lock); > __d_set_inode_and_type(tmp, inode, add_flags); > hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry); > + spin_lock(&s_anon_lock); > hlist_bl_lock(&tmp->d_sb->s_anon); > hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon); > hlist_bl_unlock(&tmp->d_sb->s_anon); > + spin_unlock(&s_anon_lock); > spin_unlock(&tmp->d_lock); > spin_unlock(&inode->i_lock); > security_d_instantiate(tmp, inode); > -- > 2.7.0 > -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Tue, Feb 02 2016, J. Bruce Fields wrote: > On Fri, Jan 29, 2016 at 11:17:43AM +1100, NeilBrown wrote: >> bit-spin-locks, as used for dcache hash chains, are not fair. >> This is not a problem for the dcache hash table as different CPUs are >> likely to access different entries in the hash table so high contention >> is not expected. >> However anonymous dentryies (created by NFSD) all live on a single hash >> chain "s_anon" and the bitlock on this can be highly contended, resulting >> in soft-lockup warnings. > > Just out of curiosity, because I can't recall seeing complaints about > warnings, when do you see it happen? Server reboot, maybe? Soft-lockup warnings. Possibly some client might notice delays longer than they should be, but the only actual complaints have been about the warnings. > > It should be hitting that __d_obtain_alias() case only when a filehandle > lookup finds a file without a cached dentry, which is an important case > to handle, but not normally what I'd expect to be the common case. Am I > forgetting something? I don't think you are missing anything significant. I too was somewhat surprised that there would be enough contention to cause problems, but the evidence was fairly conclusive (at two separate sites), and the proposed fix made the symptoms disappear. Maybe there are a great many different files being accessed and a lot of memory pressure on the server keeps pushing them out of cache. I find that customers often have loads that have quite different from what I might expect... Thanks, NeilBrown > > --b. > >> >> So introduce a global (fair) spinlock and take it before grabing the >> bitlock on s_anon. This provides fairness and makes the warnings go away. >> >> We could alternately use s_inode_list_lock, or add another spinlock >> to struct super_block. Suggestions? >> >> Signed-off-by: NeilBrown <neilb@suse.com> >> --- >> >> Dave: I'd guess you would be against using the new s_inode_list_lock >> for this because it is already highly contended - yes? >> Is it worth adding another spinlock to 'struct super_block' ? >> >> Thanks, >> NeilBrown >> >> >> fs/dcache.c | 8 ++++++++ >> 1 file changed, 8 insertions(+) >> >> diff --git a/fs/dcache.c b/fs/dcache.c >> index 92d5140de851..e83f1ac1540c 100644 >> --- a/fs/dcache.c >> +++ b/fs/dcache.c >> @@ -104,6 +104,8 @@ static unsigned int d_hash_shift __read_mostly; >> >> static struct hlist_bl_head *dentry_hashtable __read_mostly; >> >> +static DEFINE_SPINLOCK(s_anon_lock); >> + >> static inline struct hlist_bl_head *d_hash(const struct dentry *parent, >> unsigned int hash) >> { >> @@ -490,10 +492,14 @@ void __d_drop(struct dentry *dentry) >> else >> b = d_hash(dentry->d_parent, dentry->d_name.hash); >> >> + if (b == &dentry->d_sb->s_anon) >> + spin_lock(&s_anon_lock); >> hlist_bl_lock(b); >> __hlist_bl_del(&dentry->d_hash); >> dentry->d_hash.pprev = NULL; >> hlist_bl_unlock(b); >> + if (b == &dentry->d_sb->s_anon) >> + spin_unlock(&s_anon_lock); >> dentry_rcuwalk_invalidate(dentry); >> } >> } >> @@ -1978,9 +1984,11 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected) >> spin_lock(&tmp->d_lock); >> __d_set_inode_and_type(tmp, inode, add_flags); >> hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry); >> + spin_lock(&s_anon_lock); >> hlist_bl_lock(&tmp->d_sb->s_anon); >> hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon); >> hlist_bl_unlock(&tmp->d_sb->s_anon); >> + spin_unlock(&s_anon_lock); >> spin_unlock(&tmp->d_lock); >> spin_unlock(&inode->i_lock); >> security_d_instantiate(tmp, inode); >> -- >> 2.7.0 >>
On Tue, Feb 02, 2016 at 03:10:43PM +1100, NeilBrown wrote: > On Tue, Feb 02 2016, J. Bruce Fields wrote: > > > On Fri, Jan 29, 2016 at 11:17:43AM +1100, NeilBrown wrote: > >> bit-spin-locks, as used for dcache hash chains, are not fair. > >> This is not a problem for the dcache hash table as different CPUs are > >> likely to access different entries in the hash table so high contention > >> is not expected. > >> However anonymous dentryies (created by NFSD) all live on a single hash > >> chain "s_anon" and the bitlock on this can be highly contended, resulting > >> in soft-lockup warnings. > > > > Just out of curiosity, because I can't recall seeing complaints about > > warnings, when do you see it happen? Server reboot, maybe? > > Soft-lockup warnings. Possibly some client might notice delays longer > than they should be, but the only actual complaints have been about the warnings. Yeah, I was curious about the cause, not the effect. Server reboot seems like one of those cases where you might suddenly have to do a lot of uncached lookups-by-filehandle. > > > > It should be hitting that __d_obtain_alias() case only when a filehandle > > lookup finds a file without a cached dentry, which is an important case > > to handle, but not normally what I'd expect to be the common case. Am I > > forgetting something? > > I don't think you are missing anything significant. I too was somewhat > surprised that there would be enough contention to cause problems, but > the evidence was fairly conclusive (at two separate sites), and the > proposed fix made the symptoms disappear. > > Maybe there are a great many different files being accessed and a lot of > memory pressure on the server keeps pushing them out of cache. I find > that customers often have loads that have quite different from what I > might expect... Sure. Thanks for looking at this. (Feel free to add a "Reviewed-by", FWIW, though in my case that's only "agree that it fixes a real problem". I've no informed opinion on the correct solution....). --b. > > Thanks, > NeilBrown > > > > > > --b. > > > >> > >> So introduce a global (fair) spinlock and take it before grabing the > >> bitlock on s_anon. This provides fairness and makes the warnings go away. > >> > >> We could alternately use s_inode_list_lock, or add another spinlock > >> to struct super_block. Suggestions? > >> > >> Signed-off-by: NeilBrown <neilb@suse.com> > >> --- > >> > >> Dave: I'd guess you would be against using the new s_inode_list_lock > >> for this because it is already highly contended - yes? > >> Is it worth adding another spinlock to 'struct super_block' ? > >> > >> Thanks, > >> NeilBrown > >> > >> > >> fs/dcache.c | 8 ++++++++ > >> 1 file changed, 8 insertions(+) > >> > >> diff --git a/fs/dcache.c b/fs/dcache.c > >> index 92d5140de851..e83f1ac1540c 100644 > >> --- a/fs/dcache.c > >> +++ b/fs/dcache.c > >> @@ -104,6 +104,8 @@ static unsigned int d_hash_shift __read_mostly; > >> > >> static struct hlist_bl_head *dentry_hashtable __read_mostly; > >> > >> +static DEFINE_SPINLOCK(s_anon_lock); > >> + > >> static inline struct hlist_bl_head *d_hash(const struct dentry *parent, > >> unsigned int hash) > >> { > >> @@ -490,10 +492,14 @@ void __d_drop(struct dentry *dentry) > >> else > >> b = d_hash(dentry->d_parent, dentry->d_name.hash); > >> > >> + if (b == &dentry->d_sb->s_anon) > >> + spin_lock(&s_anon_lock); > >> hlist_bl_lock(b); > >> __hlist_bl_del(&dentry->d_hash); > >> dentry->d_hash.pprev = NULL; > >> hlist_bl_unlock(b); > >> + if (b == &dentry->d_sb->s_anon) > >> + spin_unlock(&s_anon_lock); > >> dentry_rcuwalk_invalidate(dentry); > >> } > >> } > >> @@ -1978,9 +1984,11 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected) > >> spin_lock(&tmp->d_lock); > >> __d_set_inode_and_type(tmp, inode, add_flags); > >> hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry); > >> + spin_lock(&s_anon_lock); > >> hlist_bl_lock(&tmp->d_sb->s_anon); > >> hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon); > >> hlist_bl_unlock(&tmp->d_sb->s_anon); > >> + spin_unlock(&s_anon_lock); > >> spin_unlock(&tmp->d_lock); > >> spin_unlock(&inode->i_lock); > >> security_d_instantiate(tmp, inode); > >> -- > >> 2.7.0 > >> -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/fs/dcache.c b/fs/dcache.c index 92d5140de851..e83f1ac1540c 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -104,6 +104,8 @@ static unsigned int d_hash_shift __read_mostly; static struct hlist_bl_head *dentry_hashtable __read_mostly; +static DEFINE_SPINLOCK(s_anon_lock); + static inline struct hlist_bl_head *d_hash(const struct dentry *parent, unsigned int hash) { @@ -490,10 +492,14 @@ void __d_drop(struct dentry *dentry) else b = d_hash(dentry->d_parent, dentry->d_name.hash); + if (b == &dentry->d_sb->s_anon) + spin_lock(&s_anon_lock); hlist_bl_lock(b); __hlist_bl_del(&dentry->d_hash); dentry->d_hash.pprev = NULL; hlist_bl_unlock(b); + if (b == &dentry->d_sb->s_anon) + spin_unlock(&s_anon_lock); dentry_rcuwalk_invalidate(dentry); } } @@ -1978,9 +1984,11 @@ static struct dentry *__d_obtain_alias(struct inode *inode, int disconnected) spin_lock(&tmp->d_lock); __d_set_inode_and_type(tmp, inode, add_flags); hlist_add_head(&tmp->d_u.d_alias, &inode->i_dentry); + spin_lock(&s_anon_lock); hlist_bl_lock(&tmp->d_sb->s_anon); hlist_bl_add_head(&tmp->d_hash, &tmp->d_sb->s_anon); hlist_bl_unlock(&tmp->d_sb->s_anon); + spin_unlock(&s_anon_lock); spin_unlock(&tmp->d_lock); spin_unlock(&inode->i_lock); security_d_instantiate(tmp, inode);
bit-spin-locks, as used for dcache hash chains, are not fair. This is not a problem for the dcache hash table as different CPUs are likely to access different entries in the hash table so high contention is not expected. However anonymous dentryies (created by NFSD) all live on a single hash chain "s_anon" and the bitlock on this can be highly contended, resulting in soft-lockup warnings. So introduce a global (fair) spinlock and take it before grabing the bitlock on s_anon. This provides fairness and makes the warnings go away. We could alternately use s_inode_list_lock, or add another spinlock to struct super_block. Suggestions? Signed-off-by: NeilBrown <neilb@suse.com> --- Dave: I'd guess you would be against using the new s_inode_list_lock for this because it is already highly contended - yes? Is it worth adding another spinlock to 'struct super_block' ? Thanks, NeilBrown fs/dcache.c | 8 ++++++++ 1 file changed, 8 insertions(+)