Re: [PATCH] livepatch: Avoid CPU hogging with cond_resched

[David Vernet <void@manifault.com>]

Adding modules + BPF list and maintainers to this thread.

David Vernet <void@manifault.com> 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@manifault.com>
> ---
>  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
>