Adding modules + BPF list and maintainers to this thread. David Vernet <void@xxxxxxxxxxxxx> wrote on Wed [2021-Dec-29 13:56:47 -0800]: > When initializing a 'struct klp_object' in klp_init_object_loaded(), and > performing relocations in klp_resolve_symbols(), klp_find_object_symbol() > is invoked to look up the address of a symbol in an already-loaded module > (or vmlinux). This, in turn, calls kallsyms_on_each_symbol() or > module_kallsyms_on_each_symbol() to find the address of the symbol that is > being patched. > > It turns out that symbol lookups often take up the most CPU time when > enabling and disabling a patch, and may hog the CPU and cause other tasks > on that CPU's runqueue to starve -- even in paths where interrupts are > enabled. For example, under certain workloads, enabling a KLP patch with > many objects or functions may cause ksoftirqd to be starved, and thus for > interrupts to be backlogged and delayed. This may end up causing TCP > retransmits on the host where the KLP patch is being applied, and in > general, may cause any interrupts serviced by softirqd to be delayed while > the patch is being applied. > > So as to ensure that kallsyms_on_each_symbol() does not end up hogging the > CPU, this patch adds a call to cond_resched() in kallsyms_on_each_symbol() > and module_kallsyms_on_each_symbol(), which are invoked when doing a symbol > lookup in vmlinux and a module respectively. Without this patch, if a > live-patch is applied on a 36-core Intel host with heavy TCP traffic, a > ~10x spike is observed in TCP retransmits while the patch is being applied. > Additionally, collecting sched events with perf indicates that ksoftirqd is > awakened ~1.3 seconds before it's eventually scheduled. With the patch, no > increase in TCP retransmit events is observed, and ksoftirqd is scheduled > shortly after it's awakened. > > Signed-off-by: David Vernet <void@xxxxxxxxxxxxx> > --- > kernel/kallsyms.c | 1 + > kernel/module.c | 2 ++ > 2 files changed, 3 insertions(+) > > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c > index 0ba87982d017..2a9afe484aec 100644 > --- a/kernel/kallsyms.c > +++ b/kernel/kallsyms.c > @@ -223,6 +223,7 @@ int kallsyms_on_each_symbol(int (*fn)(void *, const char *, struct module *, > ret = fn(data, namebuf, NULL, kallsyms_sym_address(i)); > if (ret != 0) > return ret; > + cond_resched(); > } > return 0; > } > diff --git a/kernel/module.c b/kernel/module.c > index 40ec9a030eec..c96160f7f3f5 100644 > --- a/kernel/module.c > +++ b/kernel/module.c > @@ -4462,6 +4462,8 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *, > mod, kallsyms_symbol_value(sym)); > if (ret != 0) > goto out; > + > + cond_resched(); > } > } > out: > -- > 2.30.2 >
